Random medium access methods with backoff adaptation to traffic

ABSTRACT

Using low PF values in conjunction with traffic-adapted contention windows leads to substantial decreases in delay and jitter. In general, adaptation to traffic reduces contention or delay: opening up the contention window in congestion and closing it on relief. Residual backoff adaptation provides for the reduction of the already decremented backoff values of stations that interrupted the backoff countdown process due to a transmission. It is good to adapt both the contention window and the residual backoff in order to avoid jitter. Otherwise, if the contention window is reduced but residual backoffs stay unchanged, new arrivals will enjoy shorter backoff delays than older ones, resulting in greater jitter. Adjusting both preserves the relative ordering of backoff counter values, which implies also some form of age ordering. Different adjustments can be applied to different priority traffic.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 11/239,771, filedSep. 29, 2005, now U.S. Pat. No. 7,274,708 entitled RANDOM MEDIUM ACCESSMETHODS WITH BACKOFF ADAPTATION TO TRAFFIC (currently allowed), which isa continuation of U.S. Ser. No. 10/032,507 filed on Jan. 2, 2002, (nowissued as U.S. Pat. No. 7,027,462), entitled RANDOM MEDIUM ACCESSMETHODS WITH BACKOFF ADAPTATION TO TRAFFIC, which is acontinuation-in-part of U.S. Ser. No. 09/985,257, filed Nov. 2, 2001,(now issued as U.S. Pat. No. 7,095,754), entitled, “TIERED CONTENTIONMULTIPLE ACCESS (TCMAS): A METHOD FOR PRIORITY-BASED SHARED CHANNELACCESS,” which claims the benefit of the following provisional patentapplications: U.S. Provisional Ser. No. 60/258,885, filed Jan. 2, 2001,entitled “BACKOFF WITH FAST ADAPTATION TO TRAFFIC”; U.S. ProvisionalSer. No. 60/261,165, filed Jan. 16, 2001, entitled “IMPROVED BACKOFFWITH FAST ADAPTATION TO TRAFFIC”; U.S. Provisional Ser. No. 60/264,727,filed Jan. 30, 2001, entitled, “BACKOFF WITH FAST ADAPTATION TOTRAFFIC”; U.S. Provisional Ser. No. 60/267,439, filed Feb. 9, 2001,entitled, “BACKOFF WITH FAST ADAPTATION TO TRAFFIC”; U.S. ProvisionalSer. No. 60/270,862, filed Feb. 26, 2001, entitled, “BACKOFF WITH FASTADAPTATION USING ESTIMATES OF THE NUMBER OF BACKLOGGED STATIONS”; U.S.Provisional Ser. No. 60/271,731, filed Feb. 28, 2001, entitled, “BACKOFFWITH FAST ADAPTATION USING ESTIMATES OF THE NUMBER OF BACKLOGGEDSTATIONS” and U.S. Provisional Ser. No. 60/272,786, filed Mar. 5, 2001,entitled, “IMPROVED BACKOFF WITH FAST ADAPTATION TO TRAFFIC,” all ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The invention disclosed broadly relates to telecommunications methodsand more particularly relates to collision avoidance in multiple accessnetworks. A method for backoff adaptation to traffic fluctuations isproposed for the backoff approach to collision avoidance/contentionresolution. The proposed method enables the backoff approach to maintainlow latency jitter. The adaptation procedure is generalized to includecontention-based reservation systems and contention-based media accesssystems sharing a channel with non-contention systems.

BACKGROUND OF THE INVENTION

Wireless Local Area Networks (WLANS)

Wireless local area networks (WLANs) generally operate at peak speeds ofbetween 10 to 100 Mbps and have a typical range of 100 meters.Single-cell Wireless LANs, as shown in FIG. 1A, are suitable for smallsingle-floor offices or stores. A station in a wireless LAN can be apersonal computer, a bar code scanner, or other mobile or stationarydevice that uses a wireless network interface card (NIC) to make theconnection over the RF link to other stations in the network. Thesingle-cell wireless LAN 100 of FIG. 1A provides connectivity withinradio range between wireless stations 102, 104A, 104B, 106, and 108.Access point 108 allows connections via the backbone network 110 towired network-based resources, such as servers. A single-cell wirelessLAN can typically support up to 25 users and still keep network accessdelays at an acceptable level. Multiple-cell wireless LANs providegreater range than does a single-cell, by means of a set of accesspoints and a wired network backbone to interconnect a plurality ofsingle-cell LANs. Multiple-cell wireless LANs can cover largermultiple-floor buildings. A mobile laptop computer or data collectorwith a wireless network interface card (NIC) can roam within thecoverage area while maintaining a live connection to the backbonenetwork 11.

Wireless LAN specifications and standards include the IEEE 802.11Wireless LAN Standard and the HIPERLAN Type 1 and Type 2 Standards. TheIEEE 802.11 Wireless LAN Standard is published in three parts as IEEE802.11-1999; IEEE 802.11a-1999; and IEEE 802.11b-1999, which areavailable from the IEEE, Inc. web sitehttp://grouper.ieee.org/groups/802/11. An overview of the HIPERLAN Type1 principles of operation is provided in the publication HIPERLAN Type 1Standard, ETSI ETS 300 652, WA2 December 1997. An overview of theHIPERLAN Type 2 principles of operation is provided in the BroadbandRadio Access Networks (BRAN), HIPERLAN Type 2; System Overview, ETSI TR101 683 VI.I.1 (2000-02) and a more detailed specification of itsnetwork architecture is described in HIPERLAN Type 2, Data Link Control(DLC) Layer; Part 4. Extension for Home Environment, ETSI TS 101 761-4V1.2.1 (2000-12). A subset of wireless LANs is Wireless Personal AreaNetworks (PANs), of which the Bluetooth Standard is the best known. TheBluetooth Special Interest Group, Specification Of The Bluetooth System,Version 1.1, Feb. 22, 2001, describes the principles of Bluetooth deviceoperation and communication protocols.

The IEEE 802.11 Wireless LAN Standard defines at least two differentphysical (PHY) specifications and one common medium access control (MAC)specification. The IEEE 802.11(a) Standard is designed to operate inunlicensed portions of the radio spectrum, usually either in the 2.4 GHzIndustrial, Scientific, and Medical (ISM) band or the 5 GHzUnlicensed-National Information Infrastructure (U-NII) band. It usesorthogonal frequency division multiplexing (OFDM) to deliver up to 54Mbps data rates. The IEEE 802.11(b) Standard is designed for the 2.4 GHzISM band and uses direct sequence spread spectrum (DSSS) to deliver upto 11 Mbps data rates. The IEEE 802.11 Wireless LAN Standard describestwo major components, the mobile station and the fixed access point(AP). IEEE 802.11 networks can also have an independent configurationwhere the mobile stations communicate directly with one another, withoutsupport from a fixed access point.

A single-cell wireless LAN using the IEEE 802.11 Wireless LAN Standardis an Independent Basic Service Set (IBSS) network. An IBSS has anoptional backbone network and consists of at least two wirelessstations, as shown in FIG. 1A. A multiple-cell wireless LAN using theIEEE 802.11 Wireless LAN Standard is an Extended Service Set (ESS)network. An ESS satisfies the needs of large coverage networks ofarbitrary size and complexity.

Each wireless station and access point in an IEEE 802.11 wireless LANimplements the MAC layer service, which provides the capability forwireless stations to exchange MAC frames. The MAC frame transmitsmanagement, control, or data between wireless stations and accesspoints. After a station forms the applicable MAC frame, the frame's bitsare passed to the Physical Layer for transmission.

Before transmitting a frame, the MAC layer must first gain access to thenetwork. FIG. 1B shows three interframe space (IFS) intervals that deferan IEEE 802.11 station's access to the medium and provide various levelsof priority. Each interval defines the duration between the end of thelast symbol of the previous frame 113 at time T1, to the beginning ofthe first symbol of the next frame. The Short Interframe Space (SIFS)115 provides the highest priority level by allowing some frames toaccess the medium before others, such as an Acknowledgement (ACK) frame,a Clear to Send (CTS) frame, or a subsequent fragment burst of aprevious data frame. These frames require expedited access to thenetwork to minimize frame retransmissions.

The Priority Interframe Space (PIFS) 117 of FIG. 1B is used for highpriority access to the medium during the contention-free period 116starting at T2 and ending at T3. The point coordinator 105 in the accesspoint 108 connected to backbone network 110 in FIG. 1A controls thepriority-based Point Coordination Function (PCF) to dictate whichstations in cell 100 can gain access to the medium. During thecontention-free period 116, station 102 in FIG. 1A, for example, isdirected by the access point 108 to transmit its high priority dataframe 122. The point coordinator 105 in the access point 108 sends acontention-free poll frame 120 to station 102, granting station 102permission to transmit a single frame to any destination. Station 102wants to transmit its high priority data frame 122 to the receivingstation 106. Station 102 can transmit its frame 122 during period 116 ifit senses that the medium is idle. All other stations, such as stations104A, 104B, and 106, in the cell 100 can only transmit duringcontention-free period 116 if the point coordinator grants them accessto the medium. In this example, stations 104A and 104B have low prioritydata sources 114A and 114B, and thus they must wait until the end of thecontention-free period 116 at T3. This is signaled by thecontention-free end frame 126 sent by the point coordinator in FIG. 1C.The contention-free end frame 126 is sent to identify the end of thecontention-free period 116, which occurs when time expires or when thepoint coordinator has no further frames to transmit and no stations topoll.

The distributed coordination function (DCF) Interframe Space (DIFS) 119of FIG. 1B is used by stations 104A and 104B, for example, fortransmitting low priority data frames 124A and 124B, respectively,during the contention-based period 118. The DIFS spacing delays thetransmission of lower priority frames 124A and 124B to occur between T3and T4, later than the priority-based transmission of frame 122 sent bystation 102. An Extended Interframe Space (EIFS) (not shown) goes beyondthe time of a DIFS interval as a waiting period when a bad receptionoccurs. The EIFS interval provides enough time for the receiving stationto send an acknowledgment (ACK) frame.

During the contention-based period 118, the distributed coordinationfunction (DCF) uses the Carrier-Sense Multiple Access With CollisionAvoidance (CSMA/CA) contention-based protocol, which is similar to IEEE802.3 Ethernet. The CSMA/CA protocol minimizes the chance of collisionsbetween stations sharing the medium by waiting a random backoff interval128A or 128B of FIG. 1C, if the station's sensing mechanism indicates abusy medium. The period of time immediately following traffic on themedium is when the highest probability of collisions occurs, especiallywhere there is high utilization. For example, stations 102, 104B, and106 may be waiting for the medium to become idle while station 104A istransmitting, and stations 102, 104B, and 106 will attempt to transmitat the same time, once station 104A stops. Once the medium is idle,CSMA/CA protocol causes each station to delay its transmission by arandom backoff time. For example, station 104B delays its transmissionby a random backoff time 128B, which defers station 104B fromtransmitting its frame 124B, thereby minimizing the chance it willcollide with those from other stations 102 and 106.

As shown in FIG. 1D, the CSMA/CA protocol computes the random backofftime 128B of station 104B as the product of a constant, the slot time,times a pseudo-random number RN which has a range of values from zero toa collision window CW. The value of the collision window for the firsttry to access the network by station 104B is CW1+, which yields thefirst try random backoff time 128B. If the first try to access thenetwork by station 104B fails, then the CSMA/CA protocol computes a newCW by doubling the current value of CW as CW2=CW1 times 2. As shown inFIG. 1D, the value of the collision window for the second try to accessthe network by station 104B is CW2, which yields the second try randombackoff time 128B′. This process by the CSMA/CA protocol of increasingthe delay before transmission is called binary exponential backoff. Thereason for increasing CW is to minimize collisions and maximizethroughput for both low and high network utilization. Stations with lowutilization are not forced to wait very long before transmitting theirframe. On the first or second attempt, a station will make a successfultransmission. However, if the utilization of the network is high, theCSMA/CA protocol delays stations for longer periods to avoid the chanceof multiple stations transmitting at the same time. If the second try toaccess the network by station 104B fails, then the CSMA/CA protocolcomputes a new CW by doubling again the current value of CW as CW3=CW1times 4. As shown in FIG. 1D, the value of the collision window for thethird try to access the network by station 104B is CW3, which yields thethird try random backoff time 128B″. The value of CW increases torelatively high values after successive retransmissions, under hightraffic loads. This provides greater transmission spacing betweenstations waiting to transmit.

Collision Avoidance Techniques

Four general collision avoidance approaches have emerged: [1] CarrierSense Multiple Access (CSMA) [see F. Tobagi and L. Kleinrock, “PacketSwitching in Radio Channels: Part I—Carrier Sense Multiple Access Modelsand their Throughput Delay Characteristics”, IEEE Transactions onCommunications, Vol 23, No 12, Pages 1400-1416, 1975], [2] MultipleAccess Collision Avoidance (MACA) [see P. Karn, “MACA—A New ChannelAccess Protocol for Wireless Ad-Hoc Networks”, Proceedings of theARRL/CRRL Amateur Radio Ninth Computer Networking Conference, Pages134-140, 1990], [3] their combination CSMA/CA, and [4] collisionavoidance tree expansion.

CSMA allows access attempts after sensing the channel for activity.Still, simultaneous transmit attempts lead to collisions, thus renderingthe protocol unstable at high traffic loads. The protocol also suffersfrom the hidden terminal problem.

The latter problem was resolved by the MACA protocol, which involves athree-way handshake [P. Karn, supra]. The origin node sends a request tosend (RTS) notice of the impending transmission; a response is returnedby the destination if the RTS notice is received successfully; and theorigin node proceeds with the transmission. This protocol also reducesthe average delay as collisions are detected upon transmission of merelya short message, the RTS. With the length of the packet included in theRTS and echoed in the clear to send (CTS) messages, hidden terminals canavoid colliding with the transmitted message. However, this prevents theback-to-back re-transmission in case of unsuccessfully transmittedpackets. A five-way handshake MACA protocol provides notification tocompeting sources of the successful termination of the transmission.[See V. Bharghavan, A. Demers, S. Shenker, and L. Zhang, “MACAW: A mediaaccess protocol for wireless LANs”, SIGCOMM '94, Pages 212-225, ACM,1994.]

CSMA and MACA are combined in CSMA/CA, which is MACA with carriersensing, to give better performance at high loads. A four-way handshakeis employed in the basic contention-based access protocol used in theDistributed Coordination Function (DCF) of the IEEE 802.11 Standard forWireless LANs. [See IEEE Standards Department, D3, “Wireless MediumAccess Control and Physical Layer WG,” IEEE Draft Standard P802.11Wireless LAN, January 1996.]

Collisions can be avoided by splitting the contending terminals beforetransmission is attempted. In the pseudo-Bayesian control method, eachterminal determines whether it has permission to transmit using a randomnumber generator and a permission probability “p” that depends on theestimated backlog. [See R. L. Rivest, “Network control by BayesianBroadcast”, IEEE Trans. Inform. Theory, Vol IT 25, pp. 505-515,September 1979.]

To resolve collisions, subsequent transmission attempts are typicallystaggered randomly in time using the following two approaches: binarytree and binary exponential backoff.

Upon collision, the binary tree method requires the contending nodes toself-partition into two groups with specified probabilities. Thisprocess is repeated with each new collision. The order in whichcontending nodes transmit is determined either by serial or parallelresolution of the tree. [See J. L. Massey, “Collision-resolutionalgorithms and random-access communications”, in Multi-UserCommunication Systems, G. Longo (ed.), CISM Courses and Lectures No.265, New York: Springer 1982, pp. 73-137.]

In the binary exponential backoff approach, a backoff counter tracks thenumber of pauses and hence the number of completed transmissions beforea node with pending packets attempts to seize the channel. A contendingnode initializes its backoff counter by drawing a random value, giventhe backoff window size. Each time the channel is found idle, thebackoff counter is decreased and transmission is attempted uponexpiration of the backoff counter. The window size is doubled every timea collision occurs, and the backoff countdown starts again. [See A.Tanenbaum, Computer Networks, 3^(rd) ed., Upper Saddle River, N.J.,Prentice Hall, 1996.] The Distributed Coordination Function (DCF) of theIEEE 802.11 Standard for Wireless LANs employs a variant of thiscontention resolution scheme, a truncated binary exponential backoff,starting at a specified window and allowing up to a maximum backoffrange below which transmission is attempted. [IEEE Standards Department,D3, supra] Different backoff counters may be maintained by a contendingnode for traffic to specific destinations. [Bharghavan, supra]

Quality of Service (QoS)

Quality of service (QoS) is a measure of service quality provided to acustomer. The primary measures of QoS are message loss, message delay,and network availability. Voice and video applications have the mostrigorous delay and loss requirements. Interactive data applications suchas Web browsing have less restrained delay and loss requirements, butthey are sensitive to errors. Non-real-time applications such as filetransfer, Email, and data backup operate acceptably across a wide rangeof loss rates and delay. Some applications require a minimum amount ofcapacity to operate at all, for example, voice and video. Many networkproviders guarantee specific QoS and capacity levels through the use ofService-Level Agreements (SLAs). An SLA is a contract between anenterprise user and a network provider that specifies the capacity to beprovided between points in the network that must be delivered with aspecified QoS. If the network provider fails to meet the terms of theSLA, then the user may be entitled to a refund. The SLA is typicallyoffered by network providers for private line, frame relay, ATM, orInternet networks employed by enterprises.

The transmission of time-sensitive and data application traffic over apacket network imposes requirements on the delay or delay jitter and theerror rates realized; these parameters are referred to generically asthe QoS (Quality of Service) parameters. Prioritized packet scheduling,preferential packet dropping, and bandwidth allocation are among thetechniques available at the various nodes of the network, includingaccess points, that enable packets from different applications to betreated differently, helping achieve the different quality of serviceobjectives. Such techniques exist in centralized and distributedvariations. The concern herein is with distributed mechanisms formultiple access in cellular packet networks or wireless ad hoc networks.

Backoff Adaptation to Traffic

Two general approaches can be employed for collisionavoidance/contention resolution in a contention-based medium accessprotocol, the backoff approach and the probability approach. The firstdraws a backoff counter value from a random distribution (typicallyuniform) which it counts down during idle time slots; transmission isattempted when the counter expires. In the second, transmission isattempted following each idle time slot with a fixed permissionprobability. The two approaches can be made equivalent from a channelefficiency perspective; provided, of course, their choice of parametersis consistent.

In the pseudo-Bayesian control method, the permission probability “p”used to determine whether a terminal may attempt transmission depends onthe estimated backlog. [See R. L. Rivest, “Network control by BayesianBroadcast”, IEEE Trans. Inform. Theory, Vol IT 25, pp. 505-515,September 1979.]. Ideally, the window used to draw random backoff delayson the first transmission attempt or retransmission attempt must beappropriate for the traffic intensity and contention level at hand. Theresidual backoff delays should also be adapted to traffic.

With the capability for a traffic-adapted backoff delay, a differentdiscipline may be used for adjusting the backoff window whentransmission fails. While larger backoff windows values may be usedinitially, if transmission fails, contention persistence is increased byusing a “persistence factor” less than 2 to multiply the backoff windowupon transmission retrial. This enables decreasing the mean of thestatistical distribution from which the new backoff counter would beselected for subsequent attempts. Transmission of newly arrived packetsis postponed, thus reducing the competition for the channel and giving abetter chance to aging packets to transmit successfully. The overalldelay jitter is thus minimized, making this discipline a better choicefor isochronous traffic.

Ideally one would want to start with a backoff counter appropriate forthe traffic intensity at hand and retry upon failure with successivelysmaller backoff counters in order to increase the persistence of agingpackets. The nodes can estimate traffic intensity from the number offailed transmission attempts, both their own and those of neighboringnodes. For the latter, each node includes the number of the retrialattempts in the messages exchanged during reservation and/or in thepacket headers. As each node receives these messages, it will combinethem with the retrial attempts it has experienced, assess the level ofcongestion, and select its initial backoff window accordingly. A shorterbackoff counter is needed for lower traffic intensity.

Adaptation to traffic can be pursued for both the parameters of thebackoff distribution from which backoff counters are drawn upontransmission attempt or following transmission failure, and for theadjustment of residual backoff delays. The latter is desirable in orderto preserve the age ordering implicit in the backoff approach. Olderpackets would not be put at a disadvantage relative to new arrivals ifthe latter are drawn from a backoff window shortened as a result oftraffic adaptation. The implicit preservation of age ordering achievedthrough residual backoff adaptation leads to lower delay and jitter,which are attractive for isochronous and real-time traffic streams.

SUMMARY OF THE INVENTION

In accordance with the invention, the random backoff delay used in CSMAprotocols is selected randomly from a statistical distribution, whosemean and variance are set adaptively in response to the observed trafficintensity and/or congestion estimates. The parameters of the statisticaldistribution may consist of a contention window size CW(A) which has aninitial lower value L(A) and an initial upper value U(A). Congestionestimates are derived from data that include: feedback on the success orfailure of a transmission attempt, the number the medium is idle, thenumber of re-transmissions attempted by a node and by each of itsneighbor nodes and from the age of such retrials.

Further in accordance with the invention, a new residual backoff delayis determined for a backlogged terminal/application by functions thatdepend on the traffic intensity and/or congestion estimates, and on thetime spent by the packet waiting for transmission.

Still further in accordance with the invention, adaptation to traffic isachieved through the use of “backoff scaling”. Upon arrival, or upontransmission retrial, of a packet pending transmission, a backoff delayis drawn from a traffic-adapted backoff distribution. Following a silenttime slot, a station's backoff delay is decreased and transmission isattempted upon expiration of the counter, according to the conventionalbackoff countdown procedure. In the proposed backoff procedure, iffeedback is received at a given time slot concerning traffic intensitychanges, the backoff counter is scaled up or down, depending on thedirection of the traffic change. Feedback can be based on the number ofidle time slots, the number of collisions, or other performanceparameters such as latency and dropped packets. Scaling can be appliedto both the residual backoff delay of backlogged terminals and theparameters of the random distribution (such as the offset and thecontention window size) from which backoff values are drawn for newarrivals.

Still further in accordance with the invention, several input parametersprovide differentiation between different urgency class transmissions.Differentiation between different urgency class transmissions isachieved through the use of class-specific parameters of the probabilitydistribution used to generate random backoff times and class-specificbackoff retry adjustment functions. A separate number of transmissionattempts is remembered or broadcast for each urgency class; andcongestion is thus estimated for each urgency class. This is madepossible through the introduction of new fields in all reservationmessages, including request to send (RTS) and clear to send (CTS), aswell as headers of transmitted packets. The fields indicate the numberof transmission attempts. Differentiation between different urgencyclass transmissions is achieved through the use of class-specificbackoff retry parameters. The backoff retry parameters determine how thebackoff distribution parameters are adjusted on successive retriesfollowing transmission failure. Differentiation between differenturgency class transmissions is achieved through the use of a persistencefactor, pf_(i), that is different for each class i, which is used tomultiply the backoff window from which backoff counters will be drawnrandomly upon transmission retrial.

DESCRIPTION OF THE FIGURES

FIG. 1A is a network diagram of a prior art single-cell wireless LAN,operating with the CSMA/CA protocol.

FIG. 1B is a timing diagram of the prior art CSMA/CA protocol operatingin FIG. 1A.

FIG. 1C is a more detailed timing diagram of the prior art CSMA/CAprotocol of FIG. 1B.

FIG. 1D Illustrates the prior art technique for computing the randombackoff interval in the CSMA/CA protocol of FIG. 1C.

FIG. 2 is a more detailed functional block diagram of the TCMA urgencyclass processing based and the resulting ordering of the transmission ofdata packets for three urgency classes. The urgency class processing canbe in a single wireless station with three different urgency classes orit can be distributed in multiple wireless stations, each with from oneto three urgency classes.

FIG. 3A is a more detailed functional block diagram of the TCMA urgencyclass processing of FIG. 2, showing several input parameters thatprovide differentiation between different urgency class transmissionsfor the medium urgency class for a first try backoff range L_2,U_2.

FIG. 3B is the same as FIG. 3A, but shows the resulting ordering of thetransmission of data packets for the medium urgency class for a secondtry backoff range L_2′,U_2′.

FIG. 4 is a message format diagram of a message, such as a request tosend (RTS) and clear to send (CTS), or a header of transmitted packet,indicating the number of transmission attempts. Congestion is estimatedfor each urgency class through the introduction of this message format.

FIG. 5 illustrates the backoff scaling example.

FIG. 6 illustrates backoff adaptation timing for own transmission, andgood observed transmission.

FIG. 7 illustrates backoff adaptation timing, ACK timeout, and recoveredobserved transmission failure.

FIG. 8 illustrates backoff adaptation timing, observed successful RTSand observed transmission failure.

FIG. 9 is a timing diagram of the DCF backoff procedure.

FIG. 10 is a timing diagram of priority differentiation by arbitrationtime.

FIG. 11 illustrates backoff scaling.

DISCUSSION OF THE PREFERRED EMBODIMENT

Ethernet-type random medium access methods, which rely on backoff forcollision avoidance/contention resolution, can benefit from adaptationto traffic intensity; this can be achieved through backoff scaling. Suchmethods apply to both wireless and wired media.

Scaling can be applied to both the residual backoff counters ofbacklogged terminals and the parameters of the random distribution (suchas the offset and the contention window size) from which backoff valuesare drawn for new arrivals. Suppose a traffic burst is causingcollisions and as a result the increase adjustment factor R=1 has beendetermined and supplied to the MAC sublayer of a backlogged station.Suppose there are six stations with pending packets, with counter valuesequal to: (1, 1, 1, 2, 2, 3). Suppose further that, according to thescaling procedure, each station draws the following random numbers fromthe range [0,1]: (0, 1, 0, 0, 1, 0). As shown in FIG. 4, the adjustedcounter values of the six stations become: (1, 2, 1, 3, 4, 5), whichsuggests that fewer collisions will be experienced.

In another situation, suppose that, after an interval of bursty traffic,there are sequences of idle time slots sensed on the channel. As aresult, suppose that the decrease adjustment factor D=2 is determinedand supplied to the MAC sublayer of all backlogged stations. Suppose thecounter values of three backlogged stations are: (2, 4, 7). As shown inFIG. 5, the adjusted counter values become: (1, 2, 3), leading toshorter countdown time and hence less channel idle time.

FIGS. 9, 10, and 11 illustrate the definition of the various times andtime intervals used in the scaling procedure using an example from theIEEE 802.11 EDCF protocol. Backoff adaptation provides for adjustment inthe aCWSize_(i) parameter that is advertised to the BSS by the AP in theEDCF Element of Beacon and Probe Response frames. The adjustments takeadvantage of short-term statistical fluctuations in traffic load,especially prevalent with bursty traffic, as it allows the number ofchannel idle slots to be reduced or provide a reduction in the number ofcollisions through continuous feedback and adjustment.

Given feedback concerning channel contention, the AP scales the value ofaCWSize_(i) up or down depending on the channel conditions observed.Feedback is based on the number of idle slot times, and the number ofdetected failed or successful transmissions. Based on this feedback, twoestimates are maintained as described below: the MPDU arrival rate; andthe expected number of backlogged stations having a backoff value equalto 1. If the expected number of backlogged stations having a backoffvalue equal to 1 is greater than 1, the value of aCWSize_(i) is scaledup; if this number is less than 1, aCWSize_(i) is scaled down. ESTAs settheir aCWSize_(i) to the value in the EDCF Element upon joining a BSS orIBSS or whenever they detect any change in the advertised values ofaCWSize_(i).

Scaling adjustments are made at prespecified sizes, ST_(up) for scalingup, and ST_(down) for scaling down, which could take either integer orfractional positive values. Examples would be ST_(up=)½ and ST_(down=)⅓.

The following constants can then be specified:

$\begin{matrix}{{C_{R} = {{{S\; T_{up}} + 1} = {1\frac{1}{2}}}},{and}} \\{C_{D} = {\frac{1}{{S\; T_{down}} + 1} = \frac{3}{4}}}\end{matrix}$

-   -   where, C_(R) is the scale-up factor and C_(D) is the scale-down        factor.

When the Scaling Procedure described below indicates that a scale-upfactor should be applied, the AP derives new value of aCWSize_(i) usingthe following equation:aCWSize_(i′)=trunc[C _(R)·aCWSize_(i)+0.5]

-   -   where aCWSize_(i)=current backoff window size for urgency class        i

When the Scaling Procedure described below indicates that a scale-downfactor should be applied, the AP derives new values of aCWSize_(i),using the following equation:aCWSize_(i′)=max{trunc[C _(D)·(aCWSize_(i) +ST _(down))],2}

The scaling algorithm determines whether to scale-up or down byestimating the expected number b₁ of backlogged stations having abackoff counter equal to 1, based on the observed outcome. The APmaintains an estimate of the probability p₁, which it updates byfollowing each scaling adjustment. The AP maintains also an estimate ofthe expected number n of backlogged terminals. The product n·p₁ givesb₁, the number of backlogged stations having a backoff counter equal to1.

When scaling up, p₁ is updated by dividing by the factor C_(R).

$\left. p_{1}\leftarrow\frac{p_{1}}{C_{R}} \right.$

When scaling down, p₁ is updated by dividing by the factor C_(D).

$\left. p_{1}\leftarrow\frac{p_{1}}{C_{D}} \right.$

The time to scale up or down by the specified step size ST_(up) orST_(down) is established by the following conditions. Ideally, tominimize the likelihood of a collision, the expected number ofbacklogged stations with a backoff counter of 1 must be equal to 1. Thatis, the following equation is met under optimal conditions:b ₁ =n·p ₁=1

-   -   where n is the number of backlogged stations; and

p₁ is the probability of having a backoff counter equal to 1.

The condition for scaling up by an adjustment factor ST_(up) is thefollowing:n·p ₁ ≧C _(R).

This ensures not only that b₁=n₁·p₁>1 when scaling up, but also thatscaling up by the step size ST_(up) is not excessive; that is, it wouldnot cause a change of the inequality in the opposite direction.

The condition for scaling down by an adjustment factor ST_(down) is thefollowing:n·p ₁ ≦C _(D); andn≧2.

This ensures both that b₁=n₁·p₁<1, and that a down scaling adjustment ofsize ST_(down) is not excessive and, thus, would not cause a change ofthe inequality in the opposite direction. In addition, the new backoffwindow size must be at least 2.

When the scaling conditions are met, the AP scales its aCWSize_(i)parameters, as described above. The AP sets the EDCF Element values withthe current values of aCWSize_(i) in each Beacon or Probe Response thatit transmits.

The AP bases the scaling adjustments described above on the outcome ofcontention-based transmissions. Contention based transmissions (CBTs)are all transmissions in the Contention Period that are not protected bya NAV. An observed successful transmission is any CBT that is receivedwith a good CRC. CBTs with bad CRC are also classified as successful ifthey are followed after a SIFS period by an ACK or CTS that is receivedwith a good CRC.

A value λ representing an estimate of the traffic arrival rate, is usedin adjusting the estimated number of backlogged stations. An estimate ofλ is maintained by the AP and derived from the following equation.

$\lambda = \frac{N}{\left( {T_{0} - T_{N}} \right)}$

-   -   where, N is the number of observed successful CBTs; and

T₀−T_(N) is the time elapsed for the last N successful transmissions.

Since the adjustment of aCWSize_(i), does not have immediate impact (itaffects only new arrivals or retransmissions, and not the immediatetransmissions by the backlogged terminals), the time spanned by thearrivals of the last N=20 packets would be appropriate. The AP updatesthe current λ value following each successful CBT. When a CFP occurs,updating of λ is suspended until N successful CBTs have been receivedafter the end of the CFP and, until then, the last λ value is used. TheT₀−T_(N) interval is reset at the end of each CFP.

The AP maintains a variable n₁ representing an estimate of the number ofbacklogged stations. The value of n₁ is used to decide if a scale-up orscale-down is necessary. The procedure described below is used to T_(AT)a time interval defined by two consecutive occurrences of the eventT_(AT). T_(AT) marks the end of an idle time interval of length DIFSfollowing an EIFS, an ACK or CTS timeout, or when both CCA and the NAVindicate the media is idle. The adjustment of n₁ is based on informationobtained on the event T_(δ), which is defined as the time when CCAindicates that the medium is busy following an idle period started attime T_(AT).

The AP adjusts n₁ for every idle backoff slot time. It may make thisadjustment either at the end of every backoff slot using the followingequation.n ₁ =n ₀ ·q+λ·β

-   -   where, n₀=value of n₁ in previous adjustment;

q=1−p₁; and

β=the slot time

Or, it can make one cumulative adjustment at any time up to the nextT_(AT) occurrence by determining the number t of idle slots precedingthe current transmission at time T_(δ) as follows:

$t = \frac{T_{\delta} - T_{AT}}{\beta}$

The value of n₁ is then adjusted by repeating the following equation ttimes.n ₀ =n ₁; and n ₁ =n ₀ ·q+λ·β

The AP calculates an interval δ in order to adjust the value of n₁. Theinterval δ is defined as follows:δ=T _(AT) −T _(δ)

At any time after an observed successful CBT, up to the next T_(AT)occurrence, the variable n₁ is updated based on the following “success”equation:n ₁ =n ₀ ·q+λ·(δ+β)

At any time after an observed failed CBT, up to the next T_(AT) time,the variable n₁ is updated based on the following “failure” equation:n ₁ =n ₀+2+λ·(δ+β)

Each time the variable n₁ is updated the AP sets n₀ equal to n₁.

FIG. 3A is a more detailed functional block diagram of the TCMA urgencyclass processing of FIG. 2, showing several input parameters thatprovide differentiation between different urgency class transmissions.FIG. 3A shows the resulting ordering of the transmission of data packetsfor the medium urgency class for a first try backoff range L_2,U_2.Differentiation between different urgency class transmissions isachieved through the use of the class timer 504 providing class-specificurgency arbitration times (UATs). The arbitration time is the timeinterval that the channel must be sensed idle by a node beforedecreasing its backoff counter. Initial backoff range buffer 506provides class-specific parameters of the probability distribution usedto generate random backoff times and class-specific backoff retryadjustment functions. The backoff time is drawn from a uniform randomdistribution. The backoff retry parameters determine how the backoffdistribution parameters are adjusted on successive retries followingtransmission failure. Age limit buffer 502 provides class-specificpacket age limits. The age limits lead to the cancellation of atransmission if the time since arrival exceeds a threshold value. Thepersistence factor buffer 508 provides a persistence factor, pf_(i),that is different for each class. The persistence factor, pf_(i), thatis different for each class i, will be used to multiply the backoffwindow from which backoff counters will be drawn randomly upontransmission retrial. FIG. 3A also shows the organization of the queueregister_21, the queue register_22, and the queue register_23 in theirrespectively earlier to later time order in the urgency class processing322.

FIG. 3B is the same as FIG. 3A, but shows the resulting ordering of thetransmission of data packets for the medium urgency class for a secondtry backoff range L_2′,U_2′. If the transmission is not successful, thebackoff distribution is altered before the random backoff counter ischosen for retry. The DCF doubles the backoff range (the backoff counterassumes larger values) following transmission failure. Hence, a packetis transmitted quickly in light packet traffic, but its transmission canbe delayed substantially in congestion conditions. When a traffic streamrequires low delay jitter, the goal is to minimize any deviation fromthe mean delay, which is better served if the delay is independent ofpacket-arrival rates.

The enhanced-DCF will employ a different discipline for differentclasses to adjust the backoff range when transmission fails. The initialbackoff range buffer 506 provides a new backoff range(aLower[y],aUpper[y]) which will be determined by functions that dependon the packet's class, the traffic congestion estimates, which arederived by the Traffic Intensity Estimation Procedure (TIEP), and on thetime spent by the packet waiting for transmission. These functionsdepend on the sensitivity of the class to delay or delay jitter. Thepersistence factor buffer 508 provides a persistence factor, pf_(i),that is different for each class i, which will be used to multiply thebackoff window from which backoff counters will be drawn randomly upontransmission retrial. Longer backoff ranges may be used initially fordelay jitter sensitive traffic and, if transmission fails, contentionpersistence can be increased by shifting the backoff range to lowervalues for subsequent attempts. This will have the effect of postponingtransmission and reducing the competition for the channel by new packetarrivals, giving a better chance to aging packets to transmitsuccessfully. The overall delay jitter is thus minimized, making thisdiscipline a better choice for isochronous traffic.

DETAILED DESCRIPTION OF THE INVENTION

Even though it has broader application, we describe the invention as itapplies to IEEE 802.11 wireless LANs. They have the followingcomponents. A station is any device with conformant 802.11 interfaces tothe wireless medium. A BSS (basic service set) is a set of STAscontrolled by a single coordination function. Member STAs of a BSS cancommunicate easily. An IBSS (independent basic service set) is an ad hocnetworked BSS; STAs communicate directly only. A DS (distributionsystem) provides address mapping and BSS integration into a network. AnAP (access point) is a STA that provides access to the DS.

In an IEEE 802.11 WLAN, the channel is shared by a centralized accessprotocol—the Point Coordination Function (PCF)—which providescontention-free transfer based on a polling scheme controlled by theaccess point (AP) of a basic service set (BSS). [IEEE StandardsDepartment, D3, supra] The centralized access protocol gains control ofthe channel and maintains control for the entire contention-free periodby waiting a shorter time between transmissions than the stations usingthe Distributed Coordination Function (DCF) access procedure. Thisinvention deals with the DCF.

DCF Distributed Access Mechanism

In CSMA/CA (carrier sense multiple access/collision avoidance), thechannel is sensed before transmission and backoff is chosen to postponetransmission by random amount if the medium is busy. Binary exponentialbackoff for collision resolution occurs upon collision and causes therange of backoff delay to double. The RTS/CTS (request to send/clear tosend) reservation is optional, wherein messages are exchanged to reservethe channel for point-to-point transmissions. The NAV (networkallocation vector) is used for ‘virtual’ carrier sensing. In operation,nodes other than the destination set the NAV and refrain from accessingthe medium for the NAV duration which is transmitted in the MAC. Framesare fragmented into MSDUs (MAC service data units) which are “packets”received from/delivered to the upper layers. In best-effortconnectionless user data transport, the MPDUs are transmitted asindependent entities.

DCF Backoff Procedure

A STA transmits a new frame if the medium is idle for a period>=DIFS(DCF inter-frame space). If the medium is busy, transmission is deferredfor a random backoff delay. The backoff countdown is shown in FIG. 12.The countdown starts/resumes following a busy condition after the mediumis idle for a period>=DIFS. The backoff reduces the delay by 1 for everyslot the medium is idle. The backoff is interrupted when the mediumbecomes busy. The STA transmits when backoff expires.

If the channel has been idle for a time period of length DIFS when a newframe arrives, the station may transmit immediately. However, if it isbusy, each station waits until transmission stops, and then enters intoa random backoff procedure. This prevents multiple stations from seizingthe medium immediately after completion of the preceding transmission asthey defer transmission for randomly selected time intervals. A backoffdelay is chosen randomly from a range of integers known as thecontention window. This delay measures the total idle time for which atransmission is deferred. It is expressed in units of time slots. Thelength of a time slot is sufficient to enable carrier sensing.

An internal counter is set to the selected backoff delay. The counter isreduced by 1 for every time slot the medium remains idle. Backoffcountdown is interrupted when the medium becomes busy. The countersetting is retained at the current reduced value for subsequentcountdown. Backoff countdown may not start or resume until the channelhas been idle for period of length equal to DIFS.

If the counter reaches zero, the station may begin transmission.

QoS MAC Enhancements of the DCF

The QoS-enhanced DCF schedules transmission of different types oftraffic based on their service quality specifications. The competingnodes generate various types of traffic streams that may differ by theirsensitivity to delay. Real-time traffic streams such as voice and videoare delay-sensitive with limited tolerance for long delays. Suchservices can tolerate some packet loss. Music and video on demand areexamples of isochronous traffic that tolerate longer delay but limiteddelay jitter. Their tolerance for packet loss is comparable to that ofreal-time traffic. Finally, data applications such as file transfers ore-mail are delay-insensitive but intolerant of packet loss. They arecommonly referred to as best-effort traffic.

Because of the limited tolerance for delay, the transmission ofdifferent types of packets warrants different urgency. Each nodedetermines the urgency class of its pending packets according to ascheduling algorithm. There are several urgency classes. The urgencyclass indicates the desired ordering. Pending packets in a given urgencyclass must be transmitted before transmitting packets of a lower urgencyclass. Two basic approaches can be used to impose this ordering: abackoff counter or a persistence probability.

QoS enhancements are necessary in order to facilitate streaming of voiceand multimedia traffic together with data. The high error ratesexperienced in transmitting over a wireless medium can lead to delaysand jitter that are unacceptable for such traffic. More delay is addedby acknowledgements that become necessary for wireless transmissions,and by the RTS/CTS mechanism if used.

The TCMA protocol is designed to reduce the collision probabilitybetween enhanced stations (ESTAs) of different urgency classificationaccessing a medium, at the point where collisions would most likelyoccur. Just after the medium becomes idle following a busy medium (asindicated by the clear channel assessment (CCA) function) is when thehighest probability of a collision exists. This is because multipleESTAs could have been, and with congestion will probably be, waiting forthe medium to become available again. This is the situation thatnecessitates use of TCMA, which relies on different arbitration times toprovide prioritized access to transmissions of different classification,followed by random backoff procedure to resolve medium contentionconflicts among transmissions of the same class.

TCMA Prioritization Criteria

Delay and jitter are reduced in the following ways. Frames are assignedpriority classes, which enables differential treatment of queued framesupon transmission. The priority class indicates the desired ordering oftransmission. The ordering guidelines are as follows. Frames ready fortransmission (that is, frames with expired backoff) in a given priorityclass must be transmitted before transmitting frames of a lower priorityclass. Hence, higher priority frames will be given preference over lowerpriority ones in congestion conditions (when there is high likelihood ofa higher-priority frame having a backoff time of 1). But in general, itis not desirable to postpone transmission of lower priority framesmerely because there are higher priority frames pending transmission.The latter would penalize the lower priority traffic classesexcessively.

There are nPC priority classes defined for all traffic packets which arepermanently assigned to a packet once generated; nPC=8, according toIEEE 802.1d Annex H.2. A node may generate more than one type ofpackets. When a new packet is generated at a node, it joins the pool ofpackets waiting transmission (PWT). It is assigned an urgency class.There are nUC urgency classes employed in contending for the channel.nPC and nUC are not equal; nUC is less than nPC and equal to 4.

Unlike the assignment of a priority class to a transmission, and inorder to afford the greatest flexibility, urgency classification is nottied exclusively to the traffic type; it also relies on performanceparameters as they are observed in real time. The capability to updatethe urgency class of a packet in real time is used to reflect both thepriority class of the packet and the order in which packets of differenttraffic classes and ages must be transmitted from a node. For instance,the scheduling algorithm will advance packets with short remaining lifeto a higher urgency class. For example, an isochronous applicationpacket would be placed in the buffer with a lower urgency classificationat first and then be upgraded to the same urgency as a real-timeapplication packet if its delay approaches a critical level.

Backoff Countdown Procedure

A backoff counter is employed in the same way as in binary exponentialbackoff. Typically, the backoff counter is selected randomly from arange of values, referred to as the backoff window, the reason for therandomness being to avoid collisions that would occur if more than onenode has packets awaiting transmission. The backoff counter is decreasedwhen the channel is idle for a given time interval and transmission isattempted when it expires. In case of collision, the backoff procedureis repeated up to a maximum number of times, until a specified backoffrange is reached. Once this occurs, the packet transmission iscancelled.

Backoff countdown requires that the channel is sensed for a given timeinterval, whose specification varies in different implementations of thebackoff-based collision resolution. This discussion considers twovariations of the countdown procedure: the classic backoff and thebackoff with preparation.

With classic backoff, the backoff counter is decreased when the channelis idle for a specified time interval, which is referred to as thebackoff-counter update time (BCUT). Transmission is attempted when thecounter expires. Following the transmission on the channel, a node withbackoff counter equal to 1 senses the channel, which is idle. After atime interval BCUT, the node's backoff counter begins its count down andwhen it expires, the node transmits. Transmissions by the other nodesfollow.

Backoff with preparation is a variation of the backoff described above,practiced in the IEEE 802.11 Wireless Local Area Network (WLAN) mediumaccess control (MAC) protocol. [IEEE Standards Department, D3, supra] Asin classic backoff, the backoff counter is decreased whenever thechannel is idle for a time interval equal to BCUT, except immediatelyfollowing a transmission. After a transmission is sensed on the channel,the duration of the idle required for backoff adjustment is longer; thechannel must be idle for an additional time interval, which is referredto as the backoff-counter preparation time (BCPT), before countdownstarts. Following the transmission on the channel, a node with backoffcounter equal to 1 senses the channel, which is idle. The node waits fora time interval equal to BCPT, after which the countdown procedurestarts. After a time interval BCUT, the node's backoff counter expires,and the node transmits; and other nodes follow. It is worth noting thatclassic backoff is a special case of backoff with preparation whereBCPT=0. BCPT is equal to the Distributed Coordination Functioninterframe space (DIFS), and BCUT is equal to the slot time for the IEEE802.11 Standard. [IEEE Standards Department, D3, supra]

As explained below, these basic procedures are followed, but withcertain modifications. They involve the length of the idle time intervalrequired before the backoff counter is decreased—called the arbitrationtime, the adjustment of the backoff window, and the fate of packetsreaching their transmission retrial limit.

Contention for the channel at any point in time is restricted to membersof the same urgency class, and packet transmissions are orderedaccording to their urgency class; hence the name “tiered contention”.Partitioning of contention is accomplished through the specification ofthe length of the arbitration time. The arbitration time is the timeinterval that the channel must be sensed idle by a node before startingto decrease its backoff counter. By using a different arbitration timefor each urgency class, separation of contention by urgency class isachieved. Herein, this arbitration time is called the urgencyarbitration time. Ordering of transmissions by urgency classification isaccomplished by assigning shorter arbitration times to the more urgenttraffic. This way, lower urgency packets will not cause collisions tohigher urgency packets, and will only attempt to seize the channel ifthere are no higher urgency packets pending transmission.

By assigning shorter arbitration times to higher urgency packets, higherurgency packets will dominate the channel in congestion, as lowerurgency packets would get less of a chance to decrease their backoffcounters because of their longer arbitration time. Lower urgency packetswill not cause collisions to higher urgency packets and will only beable to seize the channel if there are no higher urgency packets tryingto transmit.

Collisions between packets of different urgency classes are avoided ifthe arbitration times are selected properly. Depending on the backoffcountdown procedure employed, contention partitioning can be achievedthrough variation by urgency class of either of the two idle-timerequirements or both of them together. In other words, the urgencyarbitration time could be differentiated by one of the following: thebackoff-counter preparation time (BCPT)—yielding an urgency arbitrationtime that is equal to UAT_(i) ⁰; the backoff-counter update time(BCUT)—yielding an urgency arbitration time that is equal to UAT_(i) ¹;or both times —yielding an urgency arbitration time that is equal to thesum In the last case, when assigning urgency arbitration times toclasses, the BCUT value chosen for a lower priority class may not beless than that of higher priority class. Naturally, the differencebetween the arbitration times of two different urgency classes must beat least equal to the time necessary for a station to discern thatanother station has seized the channel.

In order to simplify the following discussion, arbitration timedifferentiation by BCPT is used.

Backoff countdown will proceed under the enhanced-DCF as under the DCF.The backoff countdown is started following a time interval during whichthe medium is determined to be idle for the duration of the UAT after atransmission. The backoff counter is decreased by 1 for each consecutivetime slot during which the medium continues to be idle.

If the medium is determined by the carrier-sense mechanism to be busy atany time during a backoff slot, then the backoff procedure is suspended;it is resumed again once the medium shall be determined to be idle forthe duration of UAT period. Transmission shall commence whenever thebackoff counter reaches zero.

It is important to recognize that the use by different urgency classesof UAT values different by aSlotTime minimizes the probability thatpackets from such classes will collide in congestion conditions; inconditions whereby several ESTAs have packets of higher classificationswith nearly expired backoff counters —that is, equal to 1—thepossibility of collision is eliminated. In such conditions, higherurgency packets will be transmitted before lower urgency packets.

Backoff Window Adjustment

Present implementations of backoff double the backoff range (the backoffcounter assumes larger values) following transmission failure. Hence, apacket is transmitted quickly in light packet traffic, but itstransmission can be delayed substantially in congestion conditions. Whena traffic stream requires low delay jitter, the goal is to minimize anydeviation from the mean delay, which is better served if the delay isindependent of packet-arrival rates. Moreover, with congestion-adaptivebackoff (see below), the random backoff values used on the firsttransmission attempt are drawn from a backoff distribution window thatis appropriate for the traffic intensity at hand. Hence, it is no longernecessary to vary the range of backoff window size widely in search of awindow size that will enable successful access at the present contentionlevel.

TCMA employs a different discipline for different classes to adjust thebackoff range when transmission fails. The new backoff range,(aLower[y],aUpper[y]), is determined by functions that depend on thepacket's class, the traffic congestion estimates, which are derived bythe Traffic Intensity Estimation Procedure (TIEP) discussed herein, andon the time spent by the packet waiting for transmission. Thesefunctions depend on the sensitivity of the class to delay or delayjitter. A persistence factor, pf_(i), that is different for each classi, will be used to multiply the backoff window from which backoffcounters will be drawn randomly upon transmission retrial. Longerbackoff ranges may be used initially for delay jitter-sensitive traffic;and if transmission fails, contention persistence could be increased byshifting the backoff range to lower values for subsequent attempts. Thiswill have the effect of postponing transmission and reducing thecompetition for the channel by new packet arrivals, giving a betterchance to aging packets to transmit successfully. The overall delayjitter is thus minimized, making this discipline a better choice forisochronous traffic.

Because of its tendency to reduce long delays, this discipline would bepreferable, in congestion conditions, to decreasing backoff ranges evenfor real-time traffic, albeit delay-sensitive. There is a tradeoff,however, as high backoff counters postpone the transmission of thepacket unnecessarily in light traffic conditions.

Class Differentiation Attributes

Both the DCF and TCMA employ CSMA/CA, with certain enhancements addedfor the latter to enable differential treatment of packets withdifferent urgency classes. The access rules used by different trafficcategories are differentiated with respect to four attributes: (1) thearbitration time, the time used for backoff countdown deferral; (2) thesize of the contention window from which the random backoff is drawn;(3) the ‘persistence factor’ used in determining the size of thecontention window in collision resolution; and (4) the MAC-layer dwelltime. The arbitration time is the time interval that the channel must besensed idle by a node before decreasing its backoff counter. Thepersistence factors is the multiplier of the initial backoff window sizeto yield the backoff window in successive retransmission attempts. Thedwell times are age limits leading to the cancellation of a transmissionif the time since arrival exceeds a threshold value.

Arbitration time is the time interval the medium must be idle before anode (queue) starts/resumes backoff countdown. If arbitration-time ispursued through the BCPT, a new IFS: AIFS (arbitration-time inter-framespace) is provided, for deferral of backoff countdown as shown in FIG.10. AIFS serves the same role as DIFS in the present standard.Higher-priority frames have a shorter AIFS.

Distinct priority treatment is achieved by AIFS lengths differing by atleast one time slot. The slot duration, which depend on the physicallayer technology, is to allow enough time for a station to sense themedium and determine whether it is busy. For example, one prioritytraffic will have an AIFS=PIFS, the next priority level will have thesame AIFS as legacy stations, namely DIFS, and lower priority levelswill have increasing AIFS length.

Because the time in which the CCA function can be completed is set atthe minimum attainable for the PHY specification, and as a resultarbitration-time differentiation alone provides for a single “high”priority class, further differentiation in priority access is pursuedthrough different backoff time distributions.

How TCMA Works

Priority differentiation by arbitration time in TCMA works in two ways.It offers not only prioritized access to frames ready for transmission,but also retards the backoff countdown process of lower-priority framesin congestion conditions. Lower-priority frames cannot countdown theirbackoff if there are higher-priority frames with backoff equal to 1waiting to transmit. This is what helps higher priority frames accessthe channel more readily in a minimally disruptive way, thus resultingin lower delays.

Consider the example of three nodes have frames queued for transmission,one each. Node A has lower priority than nodes B and C, and hence longerAIFS. At time T0, when the busy interval is over, the residual backofftimes of nodes A, B, and C are 1, 2, and 1, respectively. Following thecurrent transmission, node C waits for a time interval equal to itsAIFS, after which it starts backoff countdown. Node B does the same. Thebackoff timer of node C, starting at 1, will expire after a slot time.At that point the node transmits. The backoff of node B, which startedat 2, has become 1 by that time. Node C, which has lower priority and,hence, longer AIFS will not be able to decrement is backoff because thechannel gets busy again before it can start backoff countdown. Once thebackoff of node B expires and it transmits the queued frame, channelidle time exceeds the AIFS of node A. It can then count down its backoffand then transmit its queued frame. So, even though node B has a longerbackoff delay than node A, it transmits sooner because of its higherpriority.

Stations Generating Multiple Classes of Traffic

An example would be a PC receiving an audio-video stream and uploadingdata. Traffic generated by applications on a single station is processedas if it were generated by different stations each producing one type offrame. Separate queues are maintained for different priority classes—orcombination of classes if the number of queues is smaller than thenumber of traffic classes. Each queue follows the access rules thatapply to its class. Refinements are introduced to the protocol thatavoid collisions in a traffic-class aware manner; and reduce overhead.‘Fair’ treatment of all traffic is thus provided, as packet of a givencategory generated within the BSS will experience delays independent ofwhether the packet came from a station with a single type of packets orwith more types.

Packets generated by stations with multiple traffic types will not bedisadvantaged relative to packets from stations with a single type oftraffic because of a single contention point. Parallel queues shall bemaintained within the node for each class, each adhering to backoffprinciples consistent with that class. The only advantage enjoyed bydifferent-priority frames generated by applications in the same stationis that they do not experience inter-queue collisions, somethingotherwise possible.

The queues need not be independent, however, as packets may changequeues when their classifications are adjusted; their position in thenew queue shall be determined by the Traffic Reclassification algorithm.The transmission of packets with excessive latency is cancelled, causinga packet to leave its queue prematurely.

Each contending ESTA has access buffer of size 1. When a packet'sbackoff counter becomes 0, it shall be placed in the access buffer andattempt to seize the channel. In case of a tie, the access buffer packetwill be selected according to the urgency classification of the tiedpackets. The higher priority packet will be chosen. The packet notchosen shall follow the contention resolution procedure applicable toits class; namely, it will draw a new random backoff counter and engagein backoff countdown until its backoff counter expires. If transmissionof the chosen packet fails, it shall proceed in accordance with thecontention resolution procedure applicable to its class.

The above discussion shows that if an ESTA generates several types ofpackets, scheduling procedures internal to the ESTA will select thepacket to be transmitted. Thus, for simplicity of presentation, it isassumed in the ensuing discussion that at any point in time, an ESTA isconcerned with the transmission of packets of a single type.

TCMA Backoff Counter Distribution

An ESTA desiring to initiate transfer of data under enhanced-DCF willproceed as under DCF with some differences. The period of time requiredof the medium to be idle without interruption, as determined by thecarrier-sense mechanism, is equal to UAT, a duration that depends on thedata classification. After this UAT medium idle time, the ESTA shallthen generate a random backoff counter, unless the backoff timer alreadycontains a nonzero value.

The random backoff counter will be drawn from a uniform distributionwith range [rLower,rUpper] where the backoff window size(rUpper−rLower), or equivalently its variance ((rUpper−rLower)**2)/2, isselected based on the traffic intensity in that class. The mean of thedistribution, which is equal to (rLower+rUpper)/2, will be chosen toreflect the traffic intensity in classes of greater urgency; higherintensity in classes of greater urgency would increase the mean of thebackoff counter distribution. Traffic intensity will be estimatedthrough the Traffic Intensity Estimation Procedure (TIEP). The lowerbound of the random backoff range, rLower, will be greater than or equalto 1 for the enhanced-DCF classes with UAT=PIFS so that they do notcollide with transmissions generated by the centralized access protocol.

TCMA Overview

To summarize, the mechanism for collision resolution in TCMA employs abackoff counter resident at each node contending for the channel in thesame way as in binary exponential backoff, but with arbitration timesand persistence factors that are differentiated according to urgencyclasses. In the absence of other multiple access control protocols withwhich compatibility is sought, TCMA is described as follows:

-   -   1. An urgency class i is assigned to the packet in the access        buffer of each node according to a standardized deterministic        scheduling algorithm internal to the node.    -   2. Each node selects a backoff counter randomly from a        statistical distribution, whose mean and variance are set        adaptively in response to the observed traffic intensity.    -   3. The parameters of the backoff-counter initial distribution        are adjusted based on estimates of traffic intensity in each        urgency class.    -   4. Congestion estimates are derived from data that include:        feedback on the success or failure of a transmission attempt,        the number of re-transmissions attempted by a node and by each        of its neighbor nodes and from the age of such retrials. A        separate number of transmission attempts is remembered or        broadcast for each urgency class; and congestion is thus        estimated for each urgency class. This is made possible through        the introduction of new field in all reservation messages        [including request to send (RTS) and clear to send (CTS)] and        headers of transmitted packets indicating the number of        transmission attempts. FIG. 4 is a message format diagram of a        message, such as a request to send (RTS) and clear to send        (CTS), or a header of transmitted packet, indicating the number        of transmission attempts. Congestion is estimated for each        urgency class through the introduction of this message format.    -   5. The backoff counter would start to be decreased when the        channel is idle for a time interval equal to the arbitration        time corresponding to the urgency classification. The node would        attempt transmission when the backoff counter expires.    -   6. Upon transmission failure, new backoff distribution        parameters would be computed by decreasing the mean or by        multiplying the contention window by the persistence factor for        the urgency classification, and a new backoff counter value        would be selected from the new distribution.    -   7. The backoff procedure is repeated until a specified packet        delay is reached, which is urgency-class specific. Once this        occurs, the packet transmission is cancelled.

Congestion-Adaptive, Traffic-Specific Backoff

Ideally one would want to start with a backoff counter appropriate forthe traffic intensity at hand and retry upon failure with successivelysmaller backoff counters in order to increase the persistence of agingpackets. Adaptation to traffic is desirable in order to avoid collisionin congestion and reduce the idle time in low traffic intensity.Consistent with the notion that the Distributed Coordination Functioncould remain distributed, adaptation of the backoff distributionparameters (mean and variance) to traffic conditions will be performedin a decentralized manner, although centralized adaptation is equallyfeasible.

Adaptation of the backoff counter to traffic intensity is pursued indifferent time scales: (1) upon transmission of the packet; (2) upontransmission retrial; and (3) continuously (or whenever there is achange in traffic intensity exceeding a threshold value).

Upon arrival, or upon transmission retrial, if needed, a packet pendingtransmission draws a backoff counter value from a traffic-adaptedbackoff distribution. After every silent time slot, a packet's counteris decreased and transmission is attempted upon expiration of thecounter, according to the conventional procedure. If the trafficintensity changes, the backoff counter is scaled up or down, dependingon the direction of the traffic change, as follows.

If the traffic intensity increases, then the backoff counter isincreased relative to its current value. A random increment is selectedfrom a range (0, R), where R depends on the traffic intensity change;the increment is added to the current counter value. Countdown thenproceeds as before. By drawing the increment randomly, variation isintroduced to the new counter values of packets that had equal countervalues previously (and heading for collision), thus helping avoidcollision. This way, the relative order in which pending packets willtransmit is preserved and preference for transmission is given to olderpackets.

If the traffic intensity decreases, decreasing the backoff countervalues prevents long idle channel intervals. In order to preserve therelative time ordering of packets, a random decrement that is selectedfrom a range (0, R), which depends on the traffic intensity change, isnow subtracted from the current counter value.

By preserving the order in which pending packets will transmit, the ageof a packet is respected by the backoff approach while at the same timeallowing for quick adaptation to traffic variation. Thus it is morelikely for older packets to seize the medium before newer ones, hencekeeping the latency jitter low.

The nodes will estimate the traffic intensity from feedback informationthat includes: whether an attempted transmission succeeded or failed,the number of failed transmission attempts and the idle time spentwaiting for transmission. For the latter, each node may include in themessages exchanged during reservation and/or in the packet headers thenumber of the retrial attempts and the time since arrival of the packetat the source node. The broadcast information will be class specific,from which class-specific traffic intensity estimates will be derivedand class-specific backoff counter ranges shall be estimated.

When a node receives these messages, it will combine them with its owninformation to assess the level of congestion by the Traffic IntensityEstimation Procedure (TIEP) and select its initial backoff windowaccordingly. The adjustment of the backoff counter distributionparameters to traffic intensity shall be such that high congestion in anurgency class would increase the variance of the backoff-counterdistribution, while higher intensity in classes of greater urgency wouldincrease the mean of the backoff counter distribution.

The availability of class-specific traffic estimates will make itpossible to start with a backoff counter appropriate for the trafficintensity at hand, and retry upon failure with properly adjusted andsuccessively smaller backoff counters in order to increase thepersistence of aging packets.

In the section that follows, we present “backoff scaling”, a method foradjusting backoff window size and residual backoff delay based onobservations on the success, failure, or idle status of a channel. Westart by discussing the general case where no priorities are assigned topackets. We then give ways for dealing with prioritized traffic.

Backoff Scaling

Residual backoff and/or contention window adaptation can be done by‘backoff scaling’. This is a pseudo-Bayesian algorithm that relates tothe a-priori distribution of the expected number p of nodes, withpending transmissions, and backoff value equal to 1. p is updated basedon feedback from channel monitoring.

Ideally, one would want the estimated value of p to be 1. Nodes monitorthe medium and detect collisions/success/idle. From these observations,an observed p value is computed. An observed p value greater than 1suggests congestion. To reduce the new estimate of p to 1, the windowsize is increased by scaling up. Alternatively, if p is less than 1, onecan scale down; the window size is reduced because the present windowsize is wider than required for the present traffic. The same scalingfactor can be used to adjust residual backoff. FIG. 11 illustrates thisconcept; it shows the current and adjusted residual backoff values,respectively. When p=1.5, a node with residual backoff of 2 will scaleup to assume a value of 2, 3, or 4 with given probabilities.

Upon arrival, or upon transmission retrial, of a packet pendingtransmission, a backoff counter value is drawn from a traffic-adaptedbackoff distribution. Following a silent time slot, a station's counteris decreased and transmission is attempted upon expiration of thecounter, according to the conventional backoff procedure. In theproposed backoff procedure, if feedback is received at a given time slotconcerning traffic intensity changes, the backoff counter is scaled upor down, depending on the direction of the traffic change. Feedback canbe based on the number of idle time slots, the number of collisions, orother performance parameters such as latency and dropped packets. FIG. 5illustrates this concept; m and m′ stand for the current and adjustedbackoff counter values, respectively. The proposed traffic-adaptationmethod preserves the ordering of backoff counter values, which impliessome age ordering, thus retaining the lower latency jitter advantage ofthe backoff approach.

If the traffic intensity increases, the backoff counter is increasedrelative to its current value. A deterministic increment is added to thebackoff counter value that is proportional to R, where R is anon-negative number that depends on the traffic intensity increase. Arandom increment is selected from a range (0, R), and that, too, isadded to the current counter value. Countdown then proceeds as usual.This way, the relative order in which pending packets will transmit ispreserved. By drawing the increment randomly, variation is introduced tothe new counter values of packets that had equal counter valuespreviously (and heading for collision), thus helping avoid collision.

If the traffic intensity decreases, decreasing the backoff countervalues prevents long idle channel intervals. Again, in order to preservethe relative time ordering of packets, the current counter is scaleddown by a factor D+1, where D would be a non-negative number thatdepends on the traffic intensity decrease.

Scaling can be applied also to the parameters of the random distribution(such as the offset and the contention window size) from which backoffvalues are drawn for new arrivals. The backoff offset and window sizeare adjusted by multiplying by the factor (R+1) when scaling up, anddividing by (D+1) when scaling down.

Residual Backoff Adjustment

To illustrate the scaling adjustment in more detail, consider a stationi with a packet pending transmission that has a current backoff countervalue m_(i). It is assumed for now that the adjustment factors areinteger; fractional adjustment factors are discussed in Appendix I. Ifmany collisions occur, the adjustment factor R is estimated and used toincrease the value of the backoff counter of station i to its new valuem_(i)′ as follows:m _(i) ′=m _(i)·(1+R)−R+x _(i) =m _(i)+(m _(i)−1)R+x _(i)  (E1)

-   -   where x_(i) is an integer for station i drawn randomly from the        uniform distribution [0,R]. Because a station transmits when the        backoff counter expires, stations engaged in backoff countdown        have counter values m_(i)≧1; hence, the backoff counter will        either increase or stay at its current value after the        adjustment. FIG. 5 illustrates the possible values of m_(i)′ for        m_(i)=1,2,3 and R=1.

If long idle intervals are observed on the channel, an adjustment factorD—which is also assumed for now to be an integer—will be estimated andused to decrease the value of the backoff counter of station i to itsnew value m_(i)′ as follows:

$\begin{matrix}{m_{i}^{\prime} = {{trunc}\left\lbrack \frac{m_{i} + D}{D + 1} \right\rbrack}} & ({E2})\end{matrix}$

The function trunc[.] is applied to obtain an integer counter byrounding down. FIG. 5 illustrates the possible values of m_(i)′ form_(i)=1, . . . , 9 and D=2.

By preserving the order in which pending packets will be transmitted,the age of a packet is respected by the backoff approach, while at thesame time allowing for fast adaptation to traffic variation. Thus, thebackoff approach retains the advantage of a lower latency jitter overthe probability approach.

Backoff Distribution Parameter Adjustment

Scaling can be used for slow adaptation as well. The parameters of therandom distribution (such as the offset and the contention window size)from which backoff values are drawn for new arrivals are adjusted intandem with the residual backoff adjustment. It is assumed here, too,that the adjustment factors are integer; fractional adjustment factorsare discussed in Appendix I.

If the backoff value is drawn from a uniform distribution withparameters [L,U], the distribution parameters after scaling up become:[L′,U′], whereL′=(R+1)·L and  (E3)U′=(R+1)·U  (E4)

When the parameters of the backoff distribution are expressed in termsof an offset L and a window of size S=U−L+1, the same scalingtransformation is used. That is,S′=(R+1)·S  (E5)

The distribution parameters after scaling down become the following:

$\begin{matrix}{{L^{\prime} = {{trunc}\left\lbrack \frac{L + D}{D + 1} \right\rbrack}}{and}} & ({E6}) \\{U^{\prime} = {\max\left\{ {{{trunc}\left\lbrack \frac{U + D}{D + 1} \right\rbrack},{L^{\prime} + 1}} \right\}}} & ({E7})\end{matrix}$

In computing U′, one wants to ascertain typically that L′≠U′ afterscaling. The transformation for the window size is the following:

$\begin{matrix}{S^{\prime} = {\max\left\{ {{{trunc}\left\lbrack \frac{S + D}{D + 1} \right\rbrack},2} \right\}}} & ({E8})\end{matrix}$

Here again, one wants to ascertain typically that S≧2 after scaling. Analgorithm for scaling is presented in Appendix II.

Adaptation Based on Contention Estimate

The backoff counter would be scaled up or down by estimating theexpected number of backlogged stations having a backoff counter equal to1, based on the observed outcome. When that number gets too large, thebackoff counter is scaled up; when it gets too small, it is scaled down.

The expected number b₁ of backlogged stations with a backoff equal to 1is the product of the expected number n of backlogged stations times theprobability p₁ of having a backoff counter equal to 1. That is, b₁=n·p₁.Expansion or compression of the backoff counter changes the estimate ofp₁ as follows. When the backoff counter is scaled up by an adjustmentfactor R, p₁ decreases as follows:

$\begin{matrix}{p_{1}^{\prime} = \frac{p_{1}}{R + 1}} & ({E9})\end{matrix}$

When the backoff counter is scaled down by an adjustment factor D, p₁increases as follows:p ₁′=(D+1)·p ₁  (E10)

When scaling down the minimum backoff value for a backlogged terminal is1, which is equivalent to saying that the maximum p₁ value is also 1.Typically one would want the size of the backoff window to be at least2, which implies the following:p ₁≦1/2  (E1)

Adaptation Criteria

Ideally, we would like the expected number of backlogged stations with abackoff counter of 1 to be equal to 1. That is, we wantb=n·p ₁=1  (E12)

This ideal of making p₁ equal to 1/n is pursued through scaling thebackoff counter values up or down when the value of b is greater orsmaller than 1, respectively. That is, if b₁=n₁·p₁ is observedcurrently, the probability is adjusted through scaling to achieve theideal condition given in (E12) by dividing p₁ by b₁, as follows:

$\begin{matrix}{p_{1}^{\prime} = {\frac{p_{1}}{b_{1}} = {\frac{p_{1}}{n_{1} \cdot p_{1}} = \frac{1}{n\; 1}}}} & ({E13})\end{matrix}$

The adjustment factors would be related to b₁ as follows: R=b₁−1 whenscaling up, and

$D = {\frac{1}{b_{1}} - 1}$1 when scaling down. If the adjustment factors R and D are integer,equations (E1) through (E10) are employed. We explain in Appendix I howto adjust backoff values using fractional adjustment factors.

Of the various scaling algorithms that can be devised based on theseconcepts, we explore one that scales at fixed step sizes, which aretimed based on the current estimate of the number of backloggedterminals with backoff value equal to 1. Two equivalent approaches arepresented here.

Suppose that the adjustment factor in each direction is specified and isequal to ST; that is, R=ST and D=ST. There is a threshold value for b₁or, equivalently, for n₁ that would trigger the desired adjustment. Thethreshold value is computed as follows. Whenever b₁=n₁·p₁>1, we mustascertain that scaling up would not cause a change in the oppositedirection; b₁ or n₁ must be sufficiently large that the same inequalitywill still hold after the desired adjustment. Hence, the requirement forscaling up by an adjustment factor R=ST is that the following holds:b ₁ ≧ST+1=C _(R), or equivalently

$\begin{matrix}{{n_{1\;} \geq \frac{{S\; T} + 1}{p_{1}}} = n_{R}} & \left( {E\; 14} \right)\end{matrix}$

For any scaling down to be permitted if the new backoff window size mustbe at least 2, (see requirement in (8′)), the values of b₁ or n₁ areconstrained from above. That is,b ₁≧2·p ₁, or equivalentlyn₁≧2.  (E15)

A down scaling adjustment of D=ST is triggered when b₁=n₁·p₁<1, when b₁or n₁ are sufficiently small that the same inequality will still holdafter the desired adjustment. The requirement for scaling up by anadjustment factor R=ST is thus the following:

$\begin{matrix}\begin{matrix}{{{b_{1} \leq \frac{1}{{S\; T} + 1}} = C_{D}},} \\{{or}\mspace{14mu}{equivalently}} \\{{n_{1} \leq \frac{1}{\left( {{S\; T} + 1} \right) \cdot p_{1}}} = n_{D}}\end{matrix} & \left( {E\; 16} \right)\end{matrix}$

If we expect that traffic fluctuation is rapid, then the integer-valuedadjustment factors R and D that we have assumed so far in this sectionwill be adequate to enhance channel utilization efficiency. Otherwisefractional adjustment factors need to be applied. We address in AppendixI the use of fractional adjustment factors, which would be used for moreresponsive adaptation.

Two examples of the basic backoff adaptation algorithm are presented inAppendix II.

The discussion that follows deals with the estimation procedure for n₁(or equivalently, for b₁) used in the conditions presented above.

Pseudo-Bayesian Stabilization

To estimate the expected number of backlogged stations (or the expectednumber of backlogged stations with a backoff counter of 1), we use thefeedback obtained by a station monitoring the channel, which consists ofwhether there was an idle, success, or collision observed. During a“channel idle” interval, a station is either counting down itsbackoff—hence, the interval is known as “backoff countdown”—or has nopackets pending transmission—“station idle” interval. When the channelis busy, the station defers access—hence, we call that the “deferredaccess” interval. At the completion of the deferred access interval, thestation has feedback as to whether the transmission was successful ornot. An unsuccessful transmission on a perfect transmission medium isdue to a collision, but on a channel with interference (such as awireless channel), it could be due either to collision or tointerference. For simplicity, let us assume for now that the channel isperfect, and that the collision involves two simultaneous transmissions.

Key to this computation is the estimation of the expected number n ofbacklogged stations, which can be computed from an estimate of thetraffic arrival rate λ and the channel-monitoring outcome. The former isbased on the rate of successful transmissions observed. Variable arrivalrates are estimated from the most recent successful transmissionsobserved in a specified window of size N. The time elapsed for the lastN successful transmissions can provide an estimate of the arrival rateλ.

$\begin{matrix}{\lambda = \frac{N}{\left( {T_{0} - T_{N}} \right)}} & \left( {E\; 17} \right)\end{matrix}$

Given an estimate of λ, the expected number n of backlogged stations isupdated at every state. The system transitions into a new state when oneof the following outcomes is observed: idle, success, or failure. If n₀is the number of backlogged terminals at the time of the previousobservation (i.e. success, or failure), then the current number n₁ ofbacklogged terminals is given by relations based on concepts of thepseudo-Bayesian stabilization model presented in (2, Section 4.4.2).According to that model, if {circumflex over (n)}₀ is the estimate ofthe expected number of backlogged terminals at the time of the previousobservation, then the current estimate of the number {circumflex over(n)}₁ of backlogged terminals is given by the following relations:{circumflex over (n)} ₁ ={circumflex over (n)} ₀ ·q+λ·β for idle timeslot  (E18){circumflex over (n)} ₁ ={circumflex over (n)} ₀ ·q+λ·(π+β) forsuccess  (E19){circumflex over (n)} ₁ ={circumflex over (n)} ₀+2+λ·(π+β) forfailure  (E20)

-   -   where β is the slot time, π is the length of the transmitted        packet, and q=1−p₁.

With an imperfect channel, an unsuccessful transmission would return oneterminal to the pool of backlogged terminals if the transmission failureis due to interference; and n>1, if n packets were involved in thecollision. A way to address the ambiguity on the nature of the cause ofthe transmission failure is to use the expected number r of theterminals that would be returned to the backlogged pool, where 1≦r andtypically r≦2. Hence, (E20) can be written as follows:{circumflex over (n)} ₁ ={circumflex over (n)} ₀ +r+λ·(π+β) forfailure  (E21)

The current estimate of the expected number of backlogged terminals withbackoff value of 1, {circumflex over (b)}₁, is simply the product{circumflex over (n)}₁·p₁. So, given the estimate {circumflex over (b)}₀of the same variable at the time of the previous observation,{circumflex over (b)}₁ is given by (E18), (E19) and (E21) by simplymultiplying through by p₁, which is also estimated in tandem asdiscussed above. That is,{circumflex over (b)} ₁ ={circumflex over (b)} ₀ +q+p ₁·λ·β for idletime slot  (E22){circumflex over (b)} ₁ ={circumflex over (b)} ₀ +q+p ₁·λ·(π+β) forsuccess  (E23){circumflex over (b)} ₁ ={circumflex over (b)} ₀ +r+p ₁·λ·(π+β) forfailure  (E24)

Accommodation for 802.11 Features

The stabilization model described above accounts for the channel timespent in backoff countdown and in the contention-based transmission(CBT) following the completion of such countdown. Contention-basedtransmissions are all transmissions in the Contention Period that arenot protected by a NAV. An observed successful transmission is any CBTthat is received with a good CRC. CBTs with bad CRCs are also classifiedas successful if they are followed after a SIFS period by an ACK or CTSthat is received with a good CRC. An RTS request or a data packettransmitted without RTS are considered CBTs.

Success or failure of a CBT transmission provides information useful tothe estimation of the level of contention. The time on an 802.11channel, however, is shared by other activities as well. These includenon-contention-based transmissions (NCBT) following a reservation madeby a successful RTS request; the transmissions that follow thereservation request; contention-free use of the channel by PCF; and theinterframe spaces like SIFS, PIFS, DIFS, and EIFS used to mark thevarious activities. It is important to account for the time between thecompletion of the contention-based transmission (which follows backoffcountdown) and the start of the new backoff countdown session, as newpacket arrivals during this time will increase the number of backloggedterminals engaged in contention. We modify the above stabilization modelin order to account for these arrivals.

To accommodate these special features of 802.11, the notion of thedeferred-access interval is expanded to include the following: CBT,non-CBT, SIFS, ACK, ACKTimeout, EIFS and DIFS. FIGS. 6 through 8illustrate how the deferred-access interval is specified. For example,in FIG. 6, the deferred-access interval for a transmitting stationconsists of the sum δ=π+w, where π is the effective length of acontention-based transmission, and w is the channel time devoted toarbitration and non-contention-based transmissions, all normalized bythe packetlength and the channelrate.

The start of CBT is designated as T_(δ) and its end as T₀. T₀ occurs atthe following events:

-   -   When a station transmits a data packet, T₀ is recorded upon        receipt of an acknowledgement (ACK) or upon expiration of a        timer set equal to ACKTimeout (the sum of the duration of an        acknowledgement plus the SIFS plus the air propagation time),        whichever occurs first.    -   When a station transmits an RTS, T₀ is recorded upon receipt of        a CTS or upon expiration of a timer set equal to CTSTimeout (the        sum of the duration of a CTS plus the SIFS plus the air        propagation time), whichever occurs first.    -   When a station receives a data packet, T₀ is recorded at the end        of the busy channel time if the packet is received correctly.        Otherwise, T₀ is recorded upon receipt of an acknowledgment        (ACK), or upon expiration of a timer set equal to EIFS following        the end of the busy channel time.    -   When a station receives an RTS, CTS, data packet, or ACK, T₀ is        recorded at the at the time indicated by the NAV.

The time upon expiration of an idle time of length equal to DIFSfollowing the end of a CBT, T₀, is designated as T_(AT). A CBT startswhen the channel becomes busy for the first time after T_(AT), whichgives T_(δ). The interval π is computed as follows:π=T ₀ −T _(δ)  (E25)

The interval w starts at T₀ and ends at T_(AT). The interval w iscomputed as follows:w=T _(AT) −T ₀  (E26)

The current number {circumflex over (n)}₁ of backlogged terminalspresent after the time interval W is given by the following expression:{circumflex over (n)} ₁ ={circumflex over (n)} ₀ +λ·w or, equivalently{circumflex over (b)} ₁ ={circumflex over (b)} ₀ +p ₁ ·λ·w for the timeinterval following idle of length DIFS  (E27)

It should be noted that, because the rate of change of the expectednumber of backlogged stations is the same throughout the deferred-accessinterval, the precise designation of T₀ is not critical. As along as itoccurs between T_(π) and T_(AT), the result will be the same. If theduration of the deferred-access interval is denoted as δ, that is, ifδ=π+wthen the current estimate of the number {circumflex over (n)}₁ ofbacklogged terminals is given by the following relations:{circumflex over (n)} ₁ ={circumflex over (n)} ₀ ·q+λ·β for idle timeslot  (E28){circumflex over (n)} ₁ ={circumflex over (n)} ₀ ·q+λ·(δ+β) forsuccess  (E29){circumflex over (n)} ₁ ={circumflex over (n)} ₀ ·r+λ·(δ+β) forfailure  (E30)

The estimate of the traffic arrival rate λ is derived from the number ofobserved successful CBTs. Because of intervening CFP periods, updatingof λ is suspended until N successful CBTs have been received after theend of the CFP and, until then, the last-computed λ value is used. Theinterval T₀−T_(N) is reset at the end of each CFP.

Illustrative Implementations

Various implementations of this model are possible; we discuss twoexamples below. A variable n₀ representing an estimate of the number ofbacklogged stations is updated at discrete times. The value of n₀ isused to decide if a scale-up or scale-down is necessary. Differentimplementations of the adaptation procedure differ with respect to whenthe update occurs and when n₀ is checked in order to perform scaling.The first implementation follows the state transition updates asdescribed above, estimating the expected number of backlogged terminalsafter an idle time slot, after a CBT, and upon expiration of an idletime interval equal to DIFS. In what we call the alternativeimplementation, estimation occurs during the idle time following timeT_(AT)—the expiration of the idle time interval equal to DIFS, and uponoccurrence of the next time T_(AT). The advantages of each method willdepend on the computation time constraints, which vary with theprocessor that will be used. The implementation logic for the twoapproaches is presented below.

Scaling adjustments are made at discrete step sizes of the adjustmentfactors, triggered when the expected number of backlogged terminalsreaches a certain value or, equivalently, when the expected number ofbacklogged terminals with backoff value of 1 reaches a certain value.Multiple allowable step sizes can be selected for the adjustmentfactors. The check whether the specified threshold has beenreached—referred to below as the Scaling Check—can occur as often asevery update of the expected number of backlogged terminals, but lessfrequent checking would provide an implementation with lowercomputational load.

Each node maintains an estimate of the probability p₁, which they shallupdate following each scaling adjustment. When scaling up, it updates p₁by dividing by the factor C_(R). When scaling down, it updates p₁ bydividing by the factor C_(D).

A node maintains an estimate of the traffic arrival rate λ, which isderived from the time T₀−T_(N) elapsed for the last N successful CBTssuccessful transmissions. When a CFP occurs, update of λ is suspendeduntil N successful CBTs have been received after the end of the CFP and,until then, the last λ value shall be used. The interval T₀−T_(N) isreset at the end of each CFP.

A variable n₀ representing an estimate of the number of backloggedstations is updated at discrete times. The value of n₀ is used to decideif a scale-up or scale-down is necessary. Using the procedure describedbelow, n₀ is adjusted no later than time T_(AT), based on informationobtained on the previous times T_(δ) and T_(AT). T_(AT) marks the end ofan idle time interval of length DIFS following an EIFS, an ACK or CTStimeout, or when both CCA and the NAV indicate the medium is idle. T_(δ)is defined as the time when CCA indicates that the medium is busy forthe following idle period started at time T_(AT). n₀ is updated forevery idle backoff slot time. In the Direct implementation, thisadjustment is made at the end of every backoff slot using the equationn ₁ =n ₀ ·q+λ·β  (E31)

-   -   where q=1−p₁ and β is the slot time.

In the Alternative Implementation, one cumulative adjustment is made atany time up to the next T_(AT) time by determining the number t of idleslots preceding the current CBT at time T_(δ) as follows:

$\begin{matrix}{t = \frac{T_{\delta} - T_{A\; T}}{\beta}} & \left( {E\; 32} \right)\end{matrix}$

The value of n₀ is updated by repeating the following equation t times:n ₀ =n ₁; and n ₁=n₀ ·q+λ·β

The interval δ is calculated, in order to adjust the value of n₀, asfollows:δ=T _(AT) −T _(δ)  (E33)

At any time after an observed successful CBT, up to the next T_(AT)time, n₀ is updated based on the following equation:n ₁ =n ₀ q+λ·(δ+β) for success  (E34)

At any time after an observed failed CBT, up to the next T_(AT) time, n₀is updated based on the following equation:n ₁ =n ₀+2+λ·(δ+β) for failure  (E35)

Each time the variable n₁ is updated, n₀ is set equal to n₁.

As described above, scaling up occurs when n₁·p₁≧C_(R), while scalingdown occurs when 2≦n₁ and n₁·p₁≦C_(D).

The logic for the algorithms presented above appears on Appendix II. Thethreshold checks in the these involve the number of backloggedterminals. Equivalent algorithms can be derived by using the number ofbacklogged terminals with backoff value of 1.

Implementation Issues

In order to accommodate a variety of design considerations, we havepresented above two different implementations of a model that computesadjustment factors for fast backoff adaptation in response to feedbackon the success or failure of transmissions. In both implementations, theexpected number of backlogged stations is estimated and compared tospecified threshold values that trigger scaling.

The advantages of each method will depend on the computation time andpower constraints, which vary with the processor that will be used. Inthe alternative implementation, the update of the expected number ofbacklogged stations and its comparison for scaling action occur only atthe end of each channel-busy period, leading also to lower overallcomputation requirements. In the direct implementation several simplercomputations occur intermittently at various times between completion oftwo consecutive busy intervals. Finally, the second example of theAlternative Implementation combines the advantages of both.

The backoff distribution parameters and residual backoff counters can bescaled either by an integer or a fractional adjustment factor, thegranularity of the adjustment factors being chosen to best match trafficfluctuation. The computations for either type of factor consistprimarily of additions and multiplications or divisions by integers. Theinteger random numbers employed when scaling up need not be trulyrandom; a random sequence of integers—different for each terminal—can bestored in an array and cycled over repeatedly. The arithmetic involvingrandom numbers less than 1, which occurs when scaling by fractionaladjustment factors, can be carried out with integer random numbers,which are stored and retrieved similarly.

Priority-Class Differentiation

When differentiation between traffic classes is achieved through the useof urgency arbitration times (UATs) of different length, thedeferred-access interval remains the same for all classes; that is, itends when an idle time equal to DIFS is observed.

To meet different QoS requirements, stations generating differenttraffic streams with different transmission priorities may use differentvalues for the adjustment factors R and D for the same trafficintensity, depending on these priorities. The factor values would beselected to reflect both the traffic intensity and the transmissionpriority of the traffic stream. Traffic loads can be estimated bypriority class either by an access port, which would transmit thisinformation periodically, or by each backlogged station, which wouldread the packet header containing a field that indicates the class ofthe transmitted packet.

A multitude of class-specific adjustment factors can be derived given asystem-wide adjustment factor.

Of the many options, one must select the one that best serves QoSobjectives. Different rules are employed for deriving class-specificexpansion and compression factors, which are consistent with the backoffcounter distributions expected as a result of employing tieredcontention.

Class-Specific Expansion Factors

System-wide adjustment factors are computed using the proceduredescribed above based on the channel monitoring outcome, that is whetherthe channel is idle or whether a successful transmission or failure hasoccurred. Class-specific adjustment factors are derived from asystem-wide factor, given the breakdown of the traffic load by priorityclass, as follows.

Suppose that the backoff counter values are scaled up, due to congestionconditions when transmission failures would be observed. Let R be thefactor by which backoff counter values must be scaled up; let p₁ be theprobability computed by the adjustment algorithm above; and let λ_(i) bethe traffic load of priority i, where

${{\sum\limits_{i}\;\lambda_{i}} = \lambda},$the total load. The class-specific adjustment factors R_(i) are relatedto R as follows:

$\begin{matrix}{{\sum\limits_{i}{\lambda_{i}\frac{p_{1}^{i}}{R_{i} + 1}}} = {\lambda\frac{p_{1}}{R + 1}}} & \left( {E\; 36} \right)\end{matrix}$

-   -   where p₁ ^(i) is the proportion of backlogged stations with        traffic in class i that have residual backoff counter equal        to 1. These proportions must satisfy the following relation        after each scaling adjustment:

$\begin{matrix}{{\sum\limits_{i}\;{\lambda_{i}p_{1}^{i}}} = {\lambda\; p_{1}}} & \left( {E\; 37} \right)\end{matrix}$

Tiered contention favors high priority packets by postponing countdownof lower priority packets in congestion conditions. We would like topreserve this bias after scaling up. Hence, while the backoff countersof the higher priority traffic are scaled up by the estimatedsystem-wide expansion factor R, lower priority backoff counters could bescaled up more aggressively in order to avoid contention between themand the higher priority traffic. An approximate solution is obtained byscaling up the backoff counters of the least priority traffic [i=P] by alarger factor, while setting the top priority traffic [i=1] expansionfactor R_(i)=R. Though this approach may lead to over-expansion of thelow priority backoff counters, they will be eventually scaled down inlower traffic conditions.

Class-Specific Compression Factors

Let D be the system-wide factor by which backoff counters must be scaleddown. The class-specific compression factors D_(i) are related to D asfollows:

$\begin{matrix}{{\sum\limits_{i}\;{\lambda_{i}{p_{1}^{i}\left( {D_{i} + 1} \right)}}} = {\lambda\;{p_{1}\left( {D + 1} \right)}}} & \left( {E\; 38} \right)\end{matrix}$

Backoff counter values are typically scaled down when observingconsecutive idle time slots. Since tiered contention is designed toallow traffic of different classes to seize the channel with comparableease in such a situation, the distribution of backlogged stations withbackoff counter of 1 should not be biased in favor of a particularclass. Hence, the same compression factor is applied to all priorityclasses. That is,D_(i)=D for all i  (E39)

The above choice of Class-specific compression factors D_(i) meets therequirement in (E37).

Control Architecture

The proposed adaptation scheme can be carried out either in acentralized or distributed architecture. That is, channel monitoringwill be carried out and the adaptation computations for scaling ofeither the residual backoff counters of backlogged stations or theparameters of the distributions from which new backoff values will bedrawn can be performed by either the access port or by all the stationsand the access port together. While channel monitoring and scalingcomputations may be distributed, centralized control is used tocoordinate scaling in a BSS. The access port engages in fast adaptationcontinually and sends the required parameters and scaling commands toits associated stations continually.

With distributed control, fast adaptation may occur either continuallyif the station stays powered or only during backoff countdown, while thestation is powered and monitoring the channel. A station that has beenpowered off can obtain updated backoff distribution and scalinginformation from active stations that are engaged in countdown or fromthe access port, if present. Messages containing this information wouldbe sent by the access port to the stations regularly. Sharing ofinformation on scaling is achieved in an ad hoc network provided thatsuch information is included in a special message. We refer to thisinformation as the adaptation data. When a previously idled stationpowers on, it reads this header and is thus able to engage in adaptationwith parameters reflecting the current conditions. If a considerabletime interval of channel inactivity elapses, these parameters would berestored to the original default values.

The adaptation data broadcast under distributed control enables an idlestation to resume the adaptation calculations when it resumes countdownupon arrival of a new packet for transmission. Hence, the informationincluded in the broadcast information is the following: the estimatednumber n₁ of backlogged stations, the probability p₁ that the backoffvalue is equal to 1, and the estimated packet arrival rate λ. Thisinformation is used to estimate a new value for n₁ as it monitors thechannel during backoff countdown. In addition, the broadcast informationmust also provide the past scaling history. This history may besummarized in terms of the size of the scaled backoff distributionparameters. However, when QoS-differentiated backoff-distributionparameters are employed, the backoff distribution parameters will varypartly because of the priority class of the transmitted packet. In sucha case, a composite scaling factor, CSF, is maintained to summarize thescaling activity resulting from consecutive multiplications anddivisions by the scaling factors (R+1) and (D+1), respectively. That is,the new value of the composite scaling factor CSF′ becomes:

$\begin{matrix}{{C\; S\; F^{\prime}} = {C\; S\;{F \cdot \left( {R + 1} \right)}{following}\mspace{20mu}{backoff}\mspace{14mu}{expansion}}} & \left( {E\; 40} \right) \\{{{C\; S\; F^{\prime}} = {\frac{C\; S\; F}{D + 1}\;{following}\mspace{14mu}{backoff}\mspace{14mu}{{co}m{pression}}}}\mspace{14mu}} & \left( {E\; 41} \right)\end{matrix}$

Given this information, the current distribution parameters [L′,U′] fora packet of priority class i is obtained by multiplying the initialrange [L_(i),U_(i)] of the uniform backoff distribution for that classby the broadcast CSF value. That is,L′=trunc[CSF′·L _(i)+0.5]  (E42)U′=trunc[CSF′·U _(i)+0.5]  (E43)

The scaling factors, and the backoff values and backoff distributionparameters obtained thereof, will depend on the channel activity heardby a node (station or access port) performing the adaptation. Ingeneral, larger backoff values will result from centralized adaptationbecause the access port hears all the channel activity in a BSS, whilestations separated by a distance or other obstructions will not hear oneanother. However, stations situated in the overlapping coverage area oftwo or more access ports, or more generally stations in the periphery ofa BSS in a multi-BSS system, may hear more channel activity and/or moretransmission failures than stations closer to the access port or theaccess port itself; the former stations will select larger backoffvalues as a result.

When backoff adaptation performed by communicating nodes leads toasymmetric results—i.e., backoff values of different magnitudes—becausedifferent nodes hear different transmissions, the result is accessfailure, as a node hearing less channel activity will attempttransmission with shorter backoff values, but the destination node willnot receive that transmission because of collisions from hidden nodes.To prevent this from happening, nodes engaged in adaptation willincorporate adaptation data from other nodes and, if necessary, updatetheir own adaptation data with the more conservative values; that is,the values leading to larger backoff values. This will increase theprobability of successful access for all nodes.

Advantages of Distributed Adaptation

Distributed adaptation has the advantage that it enables monitoring ofthe channel to occur throughout the BSS and not just at a single point,the access port. This enables the system to derive adaptation data fromall nodes and process this data and perform adaptation in a way thatensures that all nodes, including the nodes experiencing the greatestamount of traffic, will be accommodated. This is important in situationsinvolving several contiguous BSSs as it can account for inter-BSSinterference. A station situated in the overlapping coverage area of twoor more access ports will perceive the channel activity affecting itstransmissions more accurately than its associated access port. By usingchannel-activity feedback on transmissions both from its own and fromthe interfering access port, such a station will adapt its backoffvalues more conservatively. Broadcasting its adaptation data to theaccess ports will cause them to expand their backoff values, thusavoiding failures.

If there are no transmissions from an access port, greater spatial reuseof the channel can be achieved in peer-to-peer communications by thestations. Since distributed adaptation permits individual stations toadapt only to the channel activity they can hear, the backoff valuesthey compute would be smaller. The access port listening to all thetransmissions in the BSS would estimate larger backoff requirements; butif the access port is not transmitting or receiving data, it will notask the stations to increase their backoff values after receiving theadaptation data broadcast by the stations. Peer-to-peer applicationsbetween physically separated stations can thus use the channelsimultaneously.

If there are transmissions from or to the access port, on the otherhand, it would indicate that larger backoff windows should be used,based on its own adaptation data. By opening up the backoff windows ofthe associated stations, the access port is not impeded from accessingthe channel.

Finally, distributed control enables an IBSS system to adapt to trafficconditions (transmission success or failure) in the absence of an accessport.

APPENDIX I Fractional Adjustment Factors

In the discussion above, integer adjustment steps were considered only.To obtain more responsive adaptation, we would like to allow forfractional adjustment steps as well.

Residual Backoff Adjustment

Scaling residual backoff values with fractional adjustment factorsproduces non-integer values which, if simply rounded, will give rise toa new distribution of backoff values that is not uniform. This reducesthe efficiency of backoff. The non-integer values obtained throughmultiplication by the scaling factor are thus rounded by probabilisticrules that ensure that the scaled residual backoff values aredistributed uniformly. We illustrate this procedure with the followingexample.

Suppose that the adjustment step ST_(up) of size ½ for scaling up andthe adjustment step ST_(down) of size ⅓ for scaling down have beenselected. The following constants are then specified:

${C_{R} = {{{ST}_{up} + 1} = {1\frac{1}{2}}}},{and}$$C_{D} = {\frac{1}{{ST}_{down} + 1} = \frac{3}{4}}$

where, C_(R) is the scale-up factor and C_(D) is the scale-down factor.

When scaling up is indicated, new values of the residual backoff m isobtained by the following computations. LetE[m′]=e+f=C _(R) ·m

where m is the residual backoff value, e is the integer part and f isthe fractional part of the product. f can be 0, or ½. Select a randomnumber x from the range [0, 1].

${{{If}\mspace{14mu} f} = 0},\begin{matrix}{{m^{\prime} = {{e - {1\mspace{14mu}{when}\mspace{14mu} x}} < \frac{1}{6}}},} \\{{m^{\prime} = {{e\mspace{14mu}{when}\mspace{14mu}\frac{1}{6}} \leq x < \frac{5}{6}}},{and}} \\{{m^{\prime} = {e + 1}},{{otherwise}.}}\end{matrix}$ ${{{If}\mspace{14mu} f} = \frac{1}{2}},\begin{matrix}{{m^{\prime} = e},{{{when}\mspace{14mu} x} < \frac{1}{2}},{and}} \\{{m^{\prime} = {e + 1}},{{otherwise}.}}\end{matrix}$

When scaling down is indicated, new values of the residual backoff m areobtained by the following computations. LetE[m′]=e+f=C _(D) ·m

where m is the residual backoff value, e is the integer part and f isthe fractional part of the product. f can be 0, ¼, ½ or ¾. Select arandom number x [0, 1].

If  f = 0, m^(′) = e${{{If}\mspace{14mu} f} = \frac{1}{4}},\begin{matrix}{{m^{\prime} = e},{{{when}\mspace{14mu} x} < \frac{5}{6}},{and}} \\{{m^{\prime} = {e + 1}},{{otherwise}.}}\end{matrix}$ ${{{If}\mspace{14mu} f} = \frac{1}{2}},\begin{matrix}{{m^{\prime} = e},{{{when}\mspace{14mu} x} < \frac{1}{2}},{and}} \\{{m^{\prime} = {e + 1}},{{otherwise}.}}\end{matrix}$ ${{{If}\mspace{14mu} f} = \frac{3}{4}},\begin{matrix}{{m^{\prime} = e},{{{when}\mspace{14mu} x} < \frac{1}{6}},{and}} \\{{m^{\prime} = {e + 1}},{{otherwise}.}}\end{matrix}$

Backoff Distribution Parameter Adjustment

The parameters of the random distribution (such as the offset and thecontention window size) from which backoff values are drawn for newarrivals are adjusted in tandem with the residual backoff adjustment. Ifthe backoff value is drawn from a uniform distribution with parameters[L,U], the distribution parameters after scaling up become: [L′,U′],whereL′=trunc[C _(R) ·L] andU′=trunc[C _(R) ·U+0.5]

When the parameters of the backoff distribution are expressed in termsof an offset L and a window of size S=U−L+1, the same scalingtransformation is used. That is,S′=trun[C _(R) ·S+0.5]

The distribution parameters after scaling down become the following:L′=max{trunc[C _(D) ·L],1} andU′=max{trunc[C _(D)·(U+ST _(down))],L′+1}

In computing U′, one wants to ascertain typically that L′≠U′ afterscaling. The transformation for the window size is the following:S′=max{trunc[C _(D)·(S+ST _(down))],2}.

Here again, one wants to ascertain typically that S≧2 after scaling. Ascaling algorithm for this example is given in Appendix II.

NUMERICAL EXAMPLE

Table 7 illustrates the mapping of old to new backoff counter values fordifferent starting backoff counter values, given the scale-up step sizeR=½.

TABLE 7 Backoff expansion transformation for fractional adjustmentfactors XIX. C_(R) m E[m′] Prob m′ 1.5 1 1.5 ½ 1 ½ 2 2 3.0 ⅙ 2 ⅓ 3 ⅙ 4 34.5 ½ 4 ½ 5 4 6.0 ⅙ 5 ⅓ 6 ⅙ 7 5 7.5 ½ 6 ½ 7 6 9.0 ⅙ 8 ⅓ 9 ⅙ 10 7 10.5 ½9 ½ 10 8 12.0 ⅙ 11 ⅓ 12 ⅙ 13 9 13.5 ½ 12 ½ 13 10 15.0 ⅙ 13 ⅓ 14 ⅙ 15

One can verify that the new backoff values m′ generated above areuniformly distributed, given that the old backoff values m were alsouniformly distributed. That is, each of the scaled values will occurwith the same probability because it will be obtained from scaling anequal proportion of the existing residual backoff values in backloggedstations.

Only backoff values greater than 1 are scaled down. Table 8 illustratesthe mapping of old to new backoff counter values for a scale-down stepsize D=⅓, for different starting backoff counter values.

TABLE 8 Backoff compression transformation for fractional adjustmentfactors XX. C_(D) m E[m′] Prob m′ 0.75 2 1.50 ½ 1 ½ 2 3 2.25 ⅚ 2 ⅙ 3 43.00 1.0 3 5 3.75 ⅙ 3 ⅚ 4 6 4.50 ½ 4 ½ 5 7 5.25 ⅚ 5 ⅙ 6 8 6.00 1.0 6 96.75 ⅙ 6 ⅚ 7 10 7.50 ½ 7 ½ 8

APPENDIX II Algorithms

Scaling Algorithms

The greater the adaptation frequency is, the more responsive theadaptation will be, leading to greater efficiency gains. On the otherhand, one wants to avoid excessive computations. Scaling can occur atspecified times or as triggered by special events. Alternatively,adjustments can be made in steps of specified size ST_(up) and ST_(down)for scaling up and down, respectively. The logic of the algorithm fortransforming the residual backoff values and the backoff distributionparameters for a fixed scaling step is given below for an integer andfractional step size.

Scaling Algorithm Logic for Integer Step Size

Determine desired scaling step size ST and specify constants

C_(R) = ST_(up) + 1, and$C_{D} = \frac{1}{\left( {{ST}_{down} + 1} \right)}$

Inputs: m=residual backoff value

(L, U)=backoff parameters

[S=backoff window size]

Block A [Scaling Up]

Step 0 Draw an integer random number x from the range [0, ST].

Step 1 Compute the new residual backoff value m′m′=m·C _(R) −ST _(up) +x

Step 2 Compute the lower bound of the scaled backoff distribution L′L′=C _(R) ·L

Step 3 Compute the upper bound of the scaled backoff distribution U′U′=C _(R) ·U

[Step 4 Compute the size of the backoff window S′S′=C _(R) ·S]

Step 4 End of Block A

Block B [Scaling Down]

Step 0 Compute the new residual backoff value m′m′=trunc[(m+ST _(down))·C _(D)]

Step 1 Compute the lower bound of the scaled backoff distribution L′L′=trunc[(L+ST _(down))·C _(D)]

Step 2 Compute the upper bound of the scaled backoff distribution U′U′=max{trunc[(U+ST _(down))·C _(D) ],L′+1}

[Step 3 Compute the size of the backoff window S′S′=max{trunc[(S+ST _(down))·S _(D)],2}]

Step 4 End of Block B

Scaling Algorithm Logic for fractional step sizes R=0.5, D=⅓

Specify constants

${C_{R} = {R + 1}},{C_{D} = \frac{1}{\left( {D + 1} \right)}},{and}$

Inputs: m=residual backoff value

(L, U)=backoff parameters

[S=backoff window size]

Block A [Scaling Up]

Step 0 Compute the expected value of m′ and determine its integer andfractional partsEm=m·C _(R) =e_integer+f_fraction

Step 1 Draw a random number x from the range [0, 1].

Step 2 If f_fraction=0, go to Step 3; otherwise go to Step 6

Step 3 If x≦⅙, set m′=e_integer−1 and go to Step 8; otherwise go to Step4

Step 4 If ⅙<x≦⅚, set m′=e_integer and go to Step 8; otherwise go to Step5

Step 5 Set m′=e_integer+1 and go to Step 8.

Step 6 If f_fraction=0.5 and x≦½ set m′=e_integer and go to Step 8;otherwise go to Step 7.

Step 7 Set m′=e_integer+1 and go to Step 8

Step 8 Compute the lower bound of the scaled backoff distribution L′L′=trunc[C _(R) ·L]

Step 9 Compute the upper bound of the scaled backoff distribution U′U′=trunc[C _(R) ·U+0.5]

[Step 10 Compute the size of the backoff window S′S′=trunc[C _(R) ·S+0.5]]

Step 11 End of Block A

Block B [Scaling Down]

Step 0 Compute the expected value of m′ and determine its integer andfractional partsEm=m·C _(D) =e_integer+f_fraction.

Step 1 Draw a random number x from the range [0, 1].

Step 2 If f_fraction=0, set m′=e_integer and go to Step 11; otherwise goto Step 3

Step 3 If f_fraction=0.25, go to Step 4; otherwise go to Step 6

Step 4 If x≦0.5, set m′=e_integer and go to Step 11; otherwise go toStep 5

Step 5 Set m′=e_integer+1 and go to Step 11

Step 6 If f_fraction=0.5, go to Step 7; otherwise go to Step 9

Step 7 If x≦½, set m′=e_integer and go to Step 11; otherwise go to Step8.

Step 8 Set m′=e_integer+1 and go to Step 11

Step 9 If x≦⅙, set m′=e_integer and go to Step 11; otherwise go to Step10

Step 10 Set m′=e_integer−1 and go to Step 11

Step 11 Compute the lower bound of the scaled backoff distribution L′L′=max{trunc[C _(D) ·L],1}

Step 12 Compute the upper bound of the scaled backoff distribution U′U′=max{trunc[C_(D)·(U+D)],L′+1}

[Step 13 Compute the size of the backoff window S′S′=max{trunc[C _(D)·(S+D)],2}]

Step 14 End of Block B

Basic Backoff Adaptation Algorithm

We present in this section two equivalent examples of the proposedalgorithm for backoff adaptation: one using estimates of b₁ and anotherusing estimates of n₁. In these examples we assume for simplicity thatthe adjustment step is the same in the two directions.

Example 1 Estimates b₁

Determine desired scaling step size ST and specify constants

C_(R) = ST + 1,  and $C_{D} = \frac{1}{\left( {{ST} + 1} \right)}$n _(start)=average·number·of·terminals

${p_{start} = \frac{1}{U + 1}};$where U is the upper bound on the starting backoff distributionb _(start) =n _(start) ·p _(start)

Algorithm Logic

Step 0 Initializen₀=n_(start);p₁=p_(start);b₀=b_(start)

Block A [Perform the Following Steps Repeatedly.]

Step 0 Estimate b₁, the number of backlogged stations with backoff valueequal to 1, using feedback data and the Estimation Procedure¹. ¹ AnEstimation Procedure is presented below.

Step 1 Invoke Block S [Scaling check]

Step 3 End of Block A

Block S [Scaling Check]

Step 0 Check whether scaling up is required; that is, if b₁≧C_(R), go toStep 1; otherwise, go to Step 2.

Step 1 Invoke the Scaling Algorithm to scale up by the adjustment factorR=ST

update p₁←p₁·C_(D);

set b₀=b₁; and

go to Step 5.

Step 2 Check whether scaling down is required; that is, if2·p₁≦b₁≦C_(D), go to Step 3; otherwise, go to Step 4.

Step 3 Invoke the Scaling Algorithm to scale down by the adjustmentfactor D=ST

update p₁←p₁·C_(R);

set b₀=b₁; and

go to Step 5.

Step 4 No scaling adjustment is made; go to Step 5

Step 5 End of Block S

Example 2 Estimates n₁

Determine desired scaling step size ST and specify constants

C_(R) = ST + 1, and $C_{D} = \frac{1}{\left( {{ST} + 1} \right)}$n _(start)=average·number·of·terminals

$p_{start} = \frac{1}{U + 1}$

where U is the upper bound on the starting backoff distribution

Algorithm Logic

Step 0 Initializen₀=n_(start);p₁=p_(start);

Block A [Perform the Following Steps Repeatedly.]

Step 0 Estimate n₁, the number of backlogged stations with backoff valueequal to 1, using feedback data and the Estimation Procedure². ² AnEstimation Procedure is presented below.

Step 1 Invoke Block S [Scaling check]

Step 2 End of Block A

Block S [Scaling Check]

Step 0 Check whether scaling up is required; that is, if n₁·p₁≧C_(R), goto Step 1; otherwise, go to Step 2.

Step 1 Invoke the Scaling Algorithm to scale up by the adjustment factorR=ST

update p₁←p₁·C_(D);

set n₀=n₁; and

go to Step 5.

Step 2 Check whether scaling down is required; that is, if 2≦n₁ andn₁·p₁≦C_(D), go to Step 3; otherwise, go to Step 4.

Step 3 Invoke the Scaling Algorithm to scale down by the adjustmentfactor D=ST

update p₁←p₁·C_(R);

set n₀=n₁; and

go to Step 5.

Step 4 No scaling adjustment is made; go to Step 5

Step 5 End of Block S

Implementations of the Backoff Adaptation Algorithm

Direct Implementation

In this implementation, the expected number of backlogged terminals isupdated after the expiration of an idle period equal to DIFS, an idletime slot, and a busy period. The feedback received upon expiration of abusy period is assumed to be whether the transmission resulted in afailure or in a successful transmission.

Example

Determine desired scaling step size ST and specify constants

C_(R) = ST + 1, and $C_{D} = \frac{1}{\left( {{ST} + 1} \right)}$n _(start)=average·number·of·terminals

${p_{start} = \frac{1}{U + 1}};$where U is the upper bound on the starting backoff distribution

$T_{st} = \frac{- N}{{aver} \cdot {input} \cdot {rate}}$

Algorithm Logic

Define q=1−p₁

Step 0 Initializen₀=n_(start);p₁=p_(start);T_(π)=0;EAT=0;T_(N)=T_(st)

[Steps 1 through 4 are repeated indefinitely.]

Step 1 Given the last T_(AT) time and n₀, invoke Block A to estimatenumber of backlogged stations n_(i) during the idle interval and invokeBlock S to perform scaling if needed

Step 2 Given n₀, invoke Block B following an idle slot to estimatenumber of backlogged stations n₁, and invoke Block S to perform scalingif needed

Step 3 Given n₀, invoke Block C following a busy channel update T₀,estimate number of backlogged stations n₁, and invoke Block S to performscaling if needed

Step 4 Go to Step 1

Block A [Determine T_(AT) and the Duration w of Non-Contention ChannelActivity.]

Step 0 Record the time T_(AT) at the end of an idle interval equal tothe arbitration time DIFS and compute w.w=T _(AT) −T ₀

Step 1 Estimate the number of backlogged stations n₁

$n_{1} = {n_{0} + {\frac{N}{T_{0} - T_{N}} \cdot w}}$

Step 2 End of Block A

Block B [At the End of an Idle Slot Time, Perform the Following Steps.]

Step 0 If the channel is sensed idle for a time slot, set the estimatednumber of backlogged stations

${n_{1} = {{n_{0} \cdot q} + {\frac{N}{T_{0} - T_{N}} \cdot \beta}}};$and go to Step 1

Step 1 End of Block B

Block C [At the End of a CBT Transmission, Perform the Following Steps.]

Step 0 Record the start T_(π) and the end T₀ of the busy channelinterval and determine ππ=T ₀ −T _(π)

Step 1 If failure was detected, set the estimated number of backloggedstations

${n_{1} = {n_{0} + r + {\frac{N}{T_{0} - T_{N}} \cdot \left( {\pi + \beta} \right)}}};{{{set}\mspace{14mu} n_{0}} = n_{1}};$and go to Step 4

Step 2 If transmission is successful, update the packet inter-arrivalrate by using a sliding window-of-N average. That is, if T_(i) is thetime of completion of the i^(th) successful transmission countingbackward from the last successful transmission,

T_(i)=T_(i-1) for i=N, . . . , 2 [once N successful transmissions havebeen observed]; and

Go to Step 3

Step 3 Set the estimated number of backlogged stations

${n_{1} = {{n_{0} \cdot q} + {\frac{N}{T_{0} - T_{N}} \cdot \left( {\pi + \beta} \right)}}};$set n₀=n₁; and go to Step 4

Step 4 Invoke Block S [Scaling check]

Step 5 End of Block C

Block S [Scaling Check]

Step 0 Check whether scaling up is required; that is, if n₁·p₁≦C_(R), goto Step 1; otherwise, go to Step 2.

Step 1 Invoke the Scaling Algorithm to scale up by the adjustment factorR=ST;

update p₁←p₁·C_(D); and

go to Step 5.

Step 2 Check whether scaling down is required; that is, if 2≦n₁ andn₁·p₁≦C_(D), go to Step 3; otherwise, go to Step 4.

Step 3 Invoke the Scaling Algorithm to scale down by the adjustmentfactor D=ST;

adjust p₁←p₁·C_(R); and

go to Step 5.

Step 4 No scaling adjustment is made; go to Step 5

Step 5 End of Block S

Alternative Implementation

A different implementation of the pseudo-Bayesian stabilization modelpresented updates the estimated number of backlogged stations only atthe end of a busy period, with the two possible outcomes being eithersuccess or failure. This implementation tracks fewer events and updatesthe estimate of the expected number of backlogged stations afterexpiration of the combined deferred-access interval. Scaling isperformed at most once per CBT. in several simple recursive computationssimilar to those used in the Direct Implementation.

The expected number n₁ of backlogged stations is updated between twoconsecutive instances of T_(AT), EAT⁻¹ and EAT₀, in two steps asfollows. Given the earlier T_(AT) time, EAT⁻¹, and the start of a CBT,T_(δ) (which is also the start of deferred access), the number t of idletime slots is computed by (E32). n₁ is updated using equation (E28) ttimes, repeatedly. This value provides n₀ for the next update. Given thestart of a CBT, T_(δ), and the later T_(AT) time, EAT₀, the length ofthe deferred-access interval δ is determined as followsδ=EAT₀ −T _(δ),  (E35)

and n₁ is updated using equations (E29) and (E30), depending on whetherthe transmission has been deemed a success or a failure. The examplebelow employs a single scaling step ST, and the Scaling Check isperformed once with each new T_(AT). The presentation of this examplemakes explicit the arithmetic efficiency of the computations.

Example

Determine desired scaling step size ST and specify constants

C_(R) = S T + 1, and $C_{D} = \frac{1}{\left( {{S\; T} + 1} \right)}$n _(start)=average·number·of·terminals

${p_{start} = \frac{1}{U + 1}};$where U is the upper bound on the starting backoff distribution

${\Delta\;{ES}_{start}} = \frac{N}{{aver} \cdot {input} \cdot {rate}}$${{constant}1} = \frac{1}{\beta}$Δn_(start)=β·(aver·input·rate)

Algorithm Logic

Define q=1−p₁

Step 0 Initializen₀=n_(start);p₁=p_(start);T_(π)=0;EAT⁻¹=0;ΔES=ΔES_(start)Δn_(idle)=Δn_(start)

[Steps 0 through 2 are repeated indefinitely.]

Step 0 Given EAT⁻¹ and n₀, invoke Block A to find the next occurrence ofT_(AT), EAT₀, and estimate number of backlogged stations n₁ at thattime.

Step 1 Invoke Block S to perform scaling if needed

Step 2 Set EAT⁻¹=EAT₀; set n₀=n₁; and go to Step 0

Block A [Estimate n₁ at the End of Deferred Access at EAT₀]

Step 0 Record the start T_(π) of a busy interval and determine thenumber of idle time slots t elapsed since the last transmission; thatis,t=(T _(δ)−EAT⁻¹)·constant1

Step 1 Estimate the number of backlogged stations n₁ by repeating ttimes the following computations:n ₀ =n ₁; and n ₁ =n ₀ q+Δn _(idle)

Step 2 Record the time EAT₀ at the end of an idle interval equal to thearbitration time DIFS and compute the increment

${\Delta\; n_{1}} = \frac{\left( {{EAT}_{0} - T_{\delta} + \beta} \right) \cdot N}{\Delta\;{ES}}$$\begin{matrix}{{Step}\mspace{14mu} 3} & {{{Set}\mspace{14mu} n_{0}} = n_{1}}\end{matrix}$

Step 4 If failure was detected, update the estimated number ofbacklogged stations as followsn ₁ =n ₀ +r+Δn ₁; set n ₀ =n ₁; and go to Step 7

Step 5 If transmission was successful, update the time between the lastN successful transmissions. That is, if ES_(i) is the T_(AT) timefollowing the completion of the i^(th) successful transmission countingbackward from the last successful transmission, update

ES_(i)=ES_(i-1) for i=N, . . . , 2 [once N successful transmissions havebeen observed]; set

ΔES=ES₀−ES_(N); compute the increment

${{\Delta\; n_{idle}} = \frac{\beta \cdot N}{\Delta\;{ES}}};$and go to Step 6

Step 6 Set the estimated number of backlogged stationsn ₁ =n ₀ q+Δn ₁; set n ₀ =n ₁; and go to Step 7

Step 7 End of Block A

Block S [Scaling Check]

Step 0 Check whether scaling up is required; that is, if n₁·p₁≧C_(R), goto Step 1; otherwise, go to Step 2.

Step 1 Invoke the Scaling Algorithm to scale up by the adjustment factorR=ST;

update p₁←p₁·C_(D); and

go to Step 5.

Step 2 Check whether scaling down is required; that is, if 2≦n₁ andn₁·p₁≦C_(D), go to Step 3; otherwise, go to Step 4.

Step 3 Invoke the Scaling Algorithm to scale down by the adjustmentfactor D=ST;

adjust p₁←p₁·C_(R); and

go to Step 5.

Step 4 No scaling adjustment is made; go to Step 5

Step 5 End of Block S

w the time between the completion of the last contention-basedtransmission

x_(i) integer for station i drawn randomly from the uniform distribution[0, R]

1. A method for a distributed medium access protocol that schedulestransmission of different types of packets on a channel based on aservice quality specification for each type of packet, comprising:determining at a plurality of nodes in a network, an urgency class foreach of a plurality of pending packets according to a schedulingalgorithm; transmitting pending packets in a given urgency class beforetransmitting packets of a lower urgency class; remembering a number oftransmission attempts by a node for a last transmission of same node;estimating from said number of transmission attempts a currentcongestion level that is experienced; and adjusting a backoff counter tothe current congestion level to provide a dispersion of packet trafficbursts.
 2. The method for a distributed medium access protocol of claim1, which further comprises: broadcasting with each transmission thenumber of transmission attempts by a node; estimating from said numberof transmission attempts received from other nodes the currentcongestion level that is experienced; and adjusting a backoff counter tothe current congestion level to provide a dispersion of packet trafficbursts.
 3. The method for a distributed medium access protocol of claim1, which further comprises: remembering the number of transmissionattempts for packets of every urgency class by a node for the a lasttransmission in that class of same node; estimating from said number oftransmission attempts the current congestion that is experienced by thean urgency class of a pending packet; and adjusting a backoff counterfor the pending packet to the current congestion level to provide adispersion of packet traffic bursts.
 4. The method for a distributedmedium access protocol of claim 1, which further comprises: broadcastingwith each transmission the number of transmission attempts by a node andthe assigned urgency class; estimating from said number of transmissionattempts received from other nodes the current congestion level that isexperienced by an urgency class of a pending packet; and adjusting abackoff counter of the pending packet to the current congestion level toprovide a dispersion of packet traffic bursts.
 5. The method for adistributed medium access protocol of claim 1, which further comprises:initializing backoff counters with a relatively longer value, and thendecreasing the value upon transmission failure and retrial.
 6. Themethod for a distributed medium access protocol of claim 1, whichfurther comprises: remembering the number of transmission attempts by anode for the last transmission of same node; estimating from said numberof transmission attempts the current congestion level that isexperienced; and adjusting a persistence probability to currentcongestion level to provide a dispersion of packet traffic bursts. 7.The method for a distributed medium access protocol of claim 1, whichfurther comprises: broadcasting with each transmission the number oftransmission attempts by a node; estimating from said number oftransmission attempts received from other nodes the current congestionlevel that is experienced; and adjusting a persistence probability tothe current congestion level to provide a dispersion of packet trafficbursts.
 8. The method for a distributed medium access protocol of claim1, which further comprises: remembering the number of transmissionattempts for packets of every urgency class by a node for a lasttransmission in that class of same node; estimating from said number oftransmission attempts the current congestion level that is experiencedby an urgency class of a pending packet; and adjusting a persistenceprobability for the pending packet to current congestion level toprovide a dispersion of packet traffic bursts.
 9. The method for adistributed medium access protocol of claim 1, which further comprises:broadcasting with each transmission the number of transmission attemptsby a node and the assigned urgency class; estimating from said number oftransmission attempts received from other nodes the current congestionlevel that is experienced by an urgency class of a pending packet; andadjusting a persistence probability of the pending packet to the currentcongestion level to provide a dispersion of packet traffic bursts. 10.The method for a distributed medium access protocol of claim 1, whichfurther comprises: initializing a persistence probability with arelatively lower value, and then increasing the value upon transmissionfailure and retrial.
 11. The method for a distributed medium accessprotocol of claim 1, which further comprises: establishing criteria forcancellation of transmission of a packet associated with packet delay.12. The method for a distributed medium access protocol of claim 1,which further comprises: applying backoff prior to attempting anytransmission.
 13. The method for a distributed medium access protocol ofclaim 1, which further comprises: checking for permission to transmit byusing a specified persistence probability prior to attempting anytransmission.
 14. The method for a distributed medium access protocol ofclaim 1, wherein said network is a wireless network.
 15. The method fora distributed medium access protocol of claim 1, wherein said network isa cellular packet network.
 16. The method for a distributed mediumaccess protocol of claim 1, wherein said network comprises one or moremulti-channel air interface systems in accordance with at least one of:Frequency Division Multiple Access (FDMA), Time Division Multiple Access(TDMA), Orthogonal Frequency Division Multiplexing (OFDM) or CodeDivision Multiple Access (CDMA).